Firefox Tweak Guide

[Page 10] Advanced Tweaking (Pt.2)



About:Config Preferences


This section contains the instructions on how to conduct advanced tweaking in Firefox using the About:Config method, as mentioned on the previous page. This is the most common way to perform advanced tweaking in Firefox, as it provides access to the widest range of settings, known as 'preferences'. To view and alter these preferences, you must first go to your Firefox Address Bar and type 'About:Config' (without quotes), then press Enter. You will be a shown a warning which you should accept, and then you will see a large listing of preferences sorted alphabetically.


There are three main types of preference variables shown here: Integers, Boolean and String variables. An Integer preference can only take a whole number value; Booleans can only take True (on) or False (off) values; and Strings can consist of any combination of text and numbers. You can change the value of any preference quickly by double-clicking on it: Booleans will toggle from True to False and back; Integers and Strings will open a dialog box which allows you to input a new value. Or you can right-click on a preference and select Modify or Toggle as relevant.


Before making any changes here, read the information on the previous page, and importantly, Please read all of the important notes below carefully before attempting to tweak your About:Config.



Click to enlarge

Important Notes


  • Some preferences are not in About:Config by default; these are denoted with a *Create tag in the list below. If you cannot find a preference mentioned below in your own About:Config listing you will have to create it as a new preference. To do this, right-click on an empty spot in the About:Config screen, select New and then select the type of preference as specified, then give it the name exactly as shown here, and an appropriate value.

  • Preferences which you have altered at any point (whether edited manually, or via the Firefox Options) will appear in bold in the About:Config screen and have a status of 'User Set'. Those which are still at their default setting will be unbolded and have the status 'Default'. To reset a preference to its default configuration, right-click on it and select Reset.

  • Only working preferences which have a noticeable impact and which cannot be adjusted within the standard Firefox settings are provided here. Some preferences which have tempting names are legacy items from previous builds of Firefox and/or do not work.

  • To undo the effects of a mistakenly entered or undesirable custom-created preference, right-click on it and select Reset, then close all instances of Firefox and restart it. However this won't remove a user-created preference from About:Config - you will need to go to your Prefs.js file (See Prefs.js on the previous page), open it with a text editor and do a search for the preference name, highlight and delete the entire line that it is on, then close and save the file. Restart Firefox, open About:Config and it should no longer be there.

  • There are far more preferences which you can change using About:Config than the ones listed here. I've only tried to cover those which I have researched, tested and believe are useful to most people. For a more complete list of Firefox preferences try the following article: Mozillazine About:Config Entries

  • To make sure your preference changes are saved and to test the effects of preference change(s), close all open instances of Firefox and restart Firefox.


    Note that aside from About:Config, there are also other About: commands you can use. Some are listed throughout this guide, but details of all of these are here.



    Preferences List


    The preferences list below is presented in alphabetical order with the full name in italics, the preference type (i.e. Integer, Boolean or String) in [ ] square brackets, whether it needs to be created denoted by *Create, then followed by a brief description. Recommendations are only given for a few of the settings - most will require you to experiment as the choice is typically based on personal taste and/or your specific hardware and OS environment settings.




    accessibility.typeaheadfind.casesensitive [Integer] - If set to 0, searches using the 'Find as you type' feature are not case sensitive, if set to 1 they are case sensitive.


    accessibility.typeaheadfind.enablesound [Boolean] - By default this is set to true, which means the 'Search for text when I start typing' feature makes a noise when you enter characters in a string which is not found on the current page. To turn this potentially annoying sound off, set this option to False.


    accessibility.typeaheadfind.linksonly [Boolean] - If set to true, this setting makes the 'Find as you type' feature search only for links on a page.


    accessibility.typeaheadfind.soundURL [String] - Determines the sound made when the 'Search for text when I start typing' feature doesn't find a match. The default is beep, but it can be set to a blank value to disable sounds, or set to the full path to a .wav file which it will play in place of the beep.


    accessibility.typeaheadfind.timeout [Integer] - Sets the amount of time before the 'Search for text when I start typing' feature search box disappears from the screen when inactive. The value is in milliseconds, so 5000 = 5 seconds for example. Lowering it will close the box more quickly when inactive.


    alerts.totalOpenTime [Integer] - Determines for how many milliseconds to show the 'Download complete' or 'Updates Available' alerts. You can increase or reduce this time from its default of 4000 = 4 seconds.


    app.update.interval [Integer] - If you've enabled automatic checking for Firefox updates in the Options menus, this setting determines how many seconds Firefox will wait between checking for updates (at the URL contained in the app.update.url preference). The default is every 24 hours (86400 seconds).


    browser.allTabs.previews [Boolean] - Disabled by default, if enabled (set to True) then a new 'List all tabs' button will be shown at the far right of the tab bar. Clicking this button will open the enhanced tab switcher. See the Tabbed Browsing area of the Features, Tips & Tricks section for details. See also the browser.ctrlTab.previews preference further below which provides another method of accessing the same functionality.


    browser.bookmarks.livemark_refresh_seconds [Integer] *Create - This is the number of seconds Firefox waits between checking for updates to any Live (RSS) Bookmarks you may have. The default is 3600 seconds = 1 hour, and for most purposes this is fine.


    browser.backspace_action [Integer] - Determines what the Backspace key does in Firefox - by default it goes back a page when pressed (a value of 0); if set to 1, it will scroll up the page when pressed; if set to 2 or above, pressing it will be disabled.


    browser.bookmarks.autoExportHTML [Boolean] - By default Firefox creates 10 bookmarks backups in .JSON format - see the previous page for details. If this setting is set to True, Firefox will start backing up bookmarks in .HTML format instead.


    browser.bookmarks.max_backups [Integer] - By default Firefox creates 10 bookmarks backups in .JSON format - see the previous page for details. The number of backups created is controlled by this setting; a value of 0 disables backups, while -1 will remove the limit on the number of backups allowed.


    browser.cache.check_doc_frequency [Integer] - This setting determines how often Firefox checks the page you're viewing against the cached version it holds. By default (a value of 3), Firefox automatically checks and only reloads the page if it seems outdated; a value of 2 will never check and always load the cached version (not recommended); a value of 1 will always check for a newer version of the page and reload; a value of 0 only checks once per session. In most cases the default of 3 is fine, but you may wish to set this to 1 if you want to ensure that Firefox always checks for and loads up the latest version of a page, though it may reduce browsing speed.


    browser.cache.disk_cache_ssl [Boolean] - When set to True, this setting allows the caching of secure web pages in your browser disk cache. This speeds up loading of commonly-visited secure pages, but is a security risk and for that reason you may wish to set this at False.


    browser.cache.memory.capacity [Integer] *Create - This setting determines whether Firefox uses system RAM to cache data, improving performance. A value of 0 turns off caching into RAM, which is not recommended as it can slow down browsing. The default value of -1 tells Firefox to automatically determine the size of the cache based on your physical RAM. Systems with 512MB of RAM will result in a 14MB cache, 1GB of RAM results in a 18MB cache, 2GB of RAM gives 24MB of cache, and 4GB of RAM or higher results in 32MB of RAM cache. You can check the amount of RAM being used on your system by typing about:cache?device=memory into the Address Bar and pressing Enter, then checking the value in the 'Maximum storage size' field. The default of -1 should be fine for all people, but if you notice Firefox increasing its memory usage for no apparent reason, you can manually restrict this value to a particular amount by entering a value in KB (e.g. a value of 18432 =18MB).


    browser.cache.memory.enable [Boolean] - This setting works in conjunction with the browser.cache.memory.capacity setting above. If set to True, Firefox will use the value specified in the browser.cache.memory.capacity setting to determine how much RAM to use. If set to False, Firefox will not use any RAM to cache, resulting in a performance drop. I recommend leaving this setting at its default of True unless troubleshooting a potential memory-related problem in Firefox for example.


    browser.cache.offline.capacity [Integer] - Web applications can be stored locally for offline use. This setting determines the maximum amount of space they can take up in the offline cache. The default value of 512000 in kilobytes corresponds to 500MB. In general there's no need to alter this as it will only be used if you install an offline web application. If you wish to turn off the offline cache altogether, set the browser.cache.offline.enable setting to False.


    browser.cache.disk.parent_directory [String] *Create - Determines the path to the parent directory for the Firefox cache, usually held under your profiles directory (i.e. under "%APPDATA%\Mozilla\"). By specifying a new path here you can move the cache to another drive or partition, which may improve performance or enhance privacy. Note that if this value is being set directly in Prefs.js or User.js, the path requires double backslashes for every backslash instead of single ones (e.g. C:\\ instead of C:\ ).


    browser.chrome.favicons [Boolean] - This setting determines the display behavior of the small site icons (usually called 'favicons') visible next to bookmarks and site URLs in the Address Bar. If set to True, these are downloaded and displayed; if set to False they are not. Turning off favicons may increase the responsiveness of the Bookmarks menu, but generally they are best left enabled for easier site identification.


    browser.chrome.image_icons.max_size [Integer] - Determines the maximum size in pixels beyond which a thumbnail of an image won't be shown in a tab; higher values can reduce performance. If you don't want thumbnail images shown in tabs at all, set this to 0.


    browser.chrome.toolbar_tips [Boolean] - If set to True, small 'tooltips' (popup boxes with descriptive text) will be shown when you hover your mouse arrow over items in your Firefox toolbar, as well as on certain objects in some webpages. If you don't like tooltips, set this value to False, but remember this may also disable useful tooltips shown on certain webpages.


    browser.ctrlTab.previews [Boolean] - Disabled by default, if enabled (set to True) then you can use the enhanced CTRL+TAB switching capabilities of Firefox. See the Tabbed Browsing area of the Features, Tips & Tricks section for full details.


    browser.display.show_image_placeholders [Boolean] - If set to True, image placeholders will be shown until images on a page are fully loaded. If set to False, these are not shown, and this can provide a cleaner look as pages load.


    browser.download.manager.flashCount [Integer] - This setting determines the number of times the Downloads window will flash in your Windows Taskbar when a download is underway and the Downloads window is minimized - by default it's twice. You can set it to 0 to disable it, or you can raise the value to increase the number of flashes if you really want it to get your attention.


    browser.download.manager.openDelay [Integer] *Create - The value here determines how many milliseconds of delay there will be before the Downloads window opens at the start of a download. By default the Download Manager window opens immediately (0 milliseconds delay), however you might like to try a value such as 1000 or 2000 (1 or 2 seconds delay) so that for very small downloads you don't have to see the Downloads window at all, yet the file will still be downloaded as usual.


    browser.download.manager.quitBehavior [Integer] - The Download Manager can pause and resume files which are being downloaded when Firefox is closed. By default (a value of 0) an active download will be automatically paused when Firefox closes, and resumed when it next opens. However you can set this to a value of 1, forcing active downloads to pause, but not automatically resume when Firefox starts up again; or set it to a value of 2 so that all active downloads will be cancelled when Firefox is closed.


    browser.download.manager.resumeOnWakeDelay [Integer] - The Download Manager resumes any active downloads when started, and this option determines the length of time in milliseconds before a download is resumed after the PC has woken from Sleep mode. You can shorten the value if your network connection is quick to recover when waking from sleep, or lengthen it to ensure the connection is active before Firefox attempts to resume.


    browser.download.manager.scanWhenDone [Boolean] - When a download is completed, and if this value is True, it will trigger an automatic scan of the download using your resident Antivirus package. However this can add a bit of time to finalizing a download, especially large downloads, so set this to False if you wish to skip this scan, but make sure to then scan any suspicious or untrusted downloads with at least one trusted malware scanners regardless.


    browser.download.manager.showAlertInterval [Integer] - This setting determines how many milliseconds pass before the 'Download Completed' alert is shown when you complete a download. If multiple downloads complete within this period of time, only one alert is shown. Default is 2 seconds (2000 milliseconds). If you want to disable the download completed alert altogether, set the browser.download.manager.showAlertOnComplete preference to False, though this isn't recommended.


    browser.enable_automatic_image_resizing [Boolean] - If set to True (the default), images opened separately will be automatically resized to fit the screen, and will need to be left-clicked to be shown at full size. If set to False they will always be shown at their full size.


    browser.fixup.alternate.enabled [Boolean] - If set to True (the default), if a web address is entered in the Address Bar without an appropriate prefix or suffix, Firefox will automatically add them in. For example if you enter http://tweakguides into the Address Bar, Firefox will add www. and .com to either side of the tweakguides text before executing the address. Setting this to False prevents that behavior, and you can also edit the prefix and suffix automatically used by Firefox in the browser.fixup.alternate.prefix and browser.fixup.alternate.suffix preferences.


    browser.fullscreen.autohide [Boolean] - If set to True (the default), when you activate Fullscreen Mode in Firefox (press F11), the toolbars and tab bar at the top of the Firefox window will automatically be hidden after a second. If set to False, they won't become hidden and will always be visible.


    browser.link.open_newwindow [Integer] - This setting determines where hyperlinks which would normally open in a new browser window end up opening. If set to 1, they open in the current tab in the existing Firefox window; if set to 2 they open in a new window; and if set to 3 (the default) all such links open in a new tab in the current Firefox window. Note that this setting is the same as that found under Tabs section of the Firefox Options>, however there is an additional choice here (setting it to 1).


    browser.link.open_newwindow.restriction [Integer] - This setting determines how new windows launched by javascript (usually popups) are treated. If set to 0, all such windows are opened as tabs in the current window; if set to 1 they open as new windows; and if set to 2 javascript windows will be treated like any other opened link unless they have strict values set. If you want to make sure popup windows do not try to hide/quietly open in the background, set this to 0 so they always open up in a new tab.


    browser.overlink-delay [Integer] - This setting determines the length of time between when you hover your mouse arrow over a hyperlink, and when the URL for that link appears in a floating popup text box at the bottom of the Firefox window. The default is almost instantaneous (70 milliseconds), but it can be lowered to 0 to make it instant, or raised to a higher value to delay its appearance or effectively disable it. New to Firefox 4.0


    browser.panorama.animate_zoom [Boolean] - If set to True (the default), when using the new Panorama feature, whenever you click on a tab or open/close Panorama, there will be an animated zoom-in/zoom-out effect. If set to False, the animated effect is removed, which can speed up the use of Panorama. New to Firefox 4.0


    browser.places.smartBookmarksVersion [Integer] - By default the Smart Bookmarks feature is disabled in Firefox. If this preference is set to a value of 0 and Firefox is restarted, then the feature will be enabled. When Smart Bookmarks is enabled, click the Bookmarks button and you will see three new folders: 'Recently Bookmarked' and 'Recent Tags' on the main Bookmarks menu, and a new 'Most Visited' folder under the 'Bookmarks Toolbar' folder. These can be manually deleted if not required by right-clicking on them and selecting Delete, and any extra separators they create can also be deleted in the Bookmarks Sidebar (press CTRL+B to open) in the same manner. See the Bookmarks section on Page 7 for more details of this feature.


    browser.preferences.animateFadeIn [Boolean] - If set to True, this setting makes the Firefox Options window automatically resize itself to fit the number of available options shown on each screen.


    browser.preferences.inContent [Boolean] - If set to True, whenever you select the normal Firefox preferences under Firefox button>Options, the preferences will open in a new tab in the browser, rather than as a separate window. New to Firefox 15.0


    browser.safebrowsing.provider. [String] - There are a range of settings here which hold the details of the URLs to be used for reporting and comparing potential malware and phishing sites to Google's Safe Browsing servers. If you wish to retain some level of malware and phishing protection, but don't want data reported back to Google, you can attempt to set the reportURL addresses to blank, however this is not recommended and will likely break the usefulness of this feature.


    browser.search.openintab [Boolean] - If set to True, the result of any searches made using the Search box in the toolbar will be shown in a newly opened tab; if set to False (the default), the search results will be shown in the current tab.


    browser.sessionhistory.max_entries [Integer] - This setting determines the total number of pages which can be stored in the page history for use with the back and forward buttons. The default value is 50, meaning up to 50 pages worth of your browsing history can be stored for a single tab - the browsing history is accessible when you right-click on the back and forward buttons.


    browser.sessionhistory.max_total_viewers [Integer] - This option determines how many pages to store in memory to speed up the back and forward buttons in Firefox. The default of -1 automatically determines the amount based on your system RAM, and is generally recommended. At 512MB of RAM, 5 pages are held in memory, while 1GB or more of RAM holds 8 pages. You can set this value to 0 to hold no pages in RAM (only recommended for very low RAM or troubleshooting purposes), or increase the value if you often use the back and forward functions for more than 8 pages.


    browser.sessionstore.max_concurrent_tabs [Integer] - This setting determines the maximum number of tabs which will actually load their content when a session is restored. The default is 3, meaning that no matter how many tabs you have restored at startup for example, only three active tabs will load into memory. Importantly, if set to 0, this will imitate the behavior of the popular BarTab add-on, which means only the active tab will be loaded into memory, reducing Firefox's overall memory usage. New to Firefox 4.0


    browser.sessionstore.max_tabs_undo [Integer] - Under the History menu in Firefox there is a 'Recently Closed Tabs' item which allows users to select from a list of recently closed tabs to reopen. The default is a history of 10 recently closed tabs, however here you can change the value to increase or decrease the number held, though it is not recommended that you set this to a high number.


    browser.sessionstore.max_windows_undo [Integer] - Under the History menu in Firefox there is a 'Recently Closed Windows' item which allows users to select from a list of recently closed Firefox windows to reopen. The default is a history of 3 recently closed windows, however here you can change the value to increase or decrease the number held, though it is not recommended that you set this to a high number.


    browser.sessionstore.resume_from_crash [Boolean] - This setting controls whether Firefox's Session Restore feature automatically restores your session if Firefox crashed or was closed improperly the last time. By default the feature is enabled, and it is recommended that you keep it that way. However if you want to disable this feature for troubleshooting purposes, set this to False.


    browser.showQuitWarning [Boolean] - Disabled by default, if set to True, and you do not have the Session Restore feature enabled, then Firefox will prompt you to save your session when you close the browser by clicking the 'Save and Quit' button. This can be useful if you want to keep any changes you've made to your Panorama groups for example. Otherwise to always enable Session Restore, select 'Show my windows and tabs from last time' in the 'Home Page' setting under the General section of the Firefox Options.


    browser.tabs.animate [Boolean] - Enabled by default, subtle tab animation features will be present, such as when you click the 'New Tab' (+) button. Set this to False if you wish to disable all such tab animation effects, which may make the interface feel snappier.


    browser.tabs.closeButtons [Integer] - This setting controls how the 'x' close buttons on tabs appear in Firefox. A value of 0 only displays the close button on the active tab; 1 (the default) shows it on all tabs; 2 results in no close buttons being shown at all; and 3 displays a single close button at the end of the tab bar.


    browser.tabs.closeWindowWithLastTab [Boolean] - By default if you close the last open tab on the screen, it will close the Firefox window altogether. If this setting is changed to False, closing the last tab will instead still leave Firefox open.


    browser.tabs.drawInTitlebar [Boolean] - If set to True, which is the default, your tab bar will be shown where the Title Bar used to be prior to Firefox 4.0, and on the same line as the Firefox button. If set to False, the tab bar will move to below the Firefox button. New to Firefox 4.0


    browser.tabs.insertRelatedAfterCurrent [Boolean] - If set to True, which is the default, whenever you open a new tab by clicking a link in an existing tab, it will open next to the current tab. If set to False, the new tab will open at the end of the tab bar.


    browser.tabs.opentabfor.middleclick [Boolean] - If set to False, clicking the middle mouse button on a link will not open that link in a tab. This is counter to the default behavior of Firefox, and would only be recommended if you strongly object to middle click opening a tab for some reason.


    browser.tabs.tabClipWidth [Integer] - Determines the minimum width of a non-active tab in pixels before a red 'x' close button appears. The default is 140, and raising this value increases the likelihood that a close button won't appear on the tab. If you want to get rid of close buttons altogether you should consider using the browser.tabs.closeButtons preference above. Note that the browser.tabs.tabminWidth and browser.tabs.tabMaxWidth preferences have been removed as of Firefox 4.0. See the Tabbed Browsing area of the Features, Tips & Tricks section for one way of altering default tab widths.


    browser.taskbar.previews.max [Integer] - This setting determines the maximum number of thumbnail previews which can be displayed in the Windows Taskbar on Vista/7 when the 'Show tab previews in the Windows taskbar' setting is enabled in the Firefox options. The default is 20, but it will also be affected by the amount of viewable space you have on your Desktop.


    browser.urlbar.clickSelectsAll [Boolean] - When set to True (the default), left-clicking once in the Address Bar automatically highlights all of its contents. If set to False, clicking in the Address Bar will only place a cursor where you click without highlighting any text first.


    browser.urlbar.maxRichResults [Integer] - This setting determines the maximum number of results shown when using the Awesome Bar functionality. The default is 12, but if you find that excessive you can lower the value here. A value of 0 will completely disable the Address Bar's 'Awesome Bar'-related functionality.


    browser.zoom.siteSpecific [Boolean] - In conjunction with Firefox's full page zooming capability, this setting determines whether your zoomed in/out view applies to the entire site (when set to True, the default), or if it only applies to the currently viewed page (when set to False).



    The next page continues the alphabetical listing of About:Config Preferences.