From MozillaZine Knowledge Base

This is not the place to edit your configuration settings. This is a reference to the entries in about:config, where all user preferences can be viewed and modified. In most cases the list below doesn't state whether a given preference applies to all, or only some (and which), of Firefox, Thunderbird, Mozilla Suite, SeaMonkey, or even the now discontinued Sunbird: trial and error is often the only way to tell if some particular preference applies to your version of your application.

Users not already familiar with about:config entries should read the companion article, about:config, which describes how to modify values and work with the about:config list.

This article does not contain a complete list of all preferences. If you don't find an entry below for a given preference, check the Category:Preferences (multiple-page) listing, which includes many new preferences that have not been added here, as well as those preferences that have been migrated from sections of this article. Also check Mail and news settings for a listing of Thunderbird and Mozilla Suite/SeaMonkey mail and news preferences. Alternative lists can be found here:

• the Preferential project
• Mozilla's embedding documentation
• Preferences - MDN
• Mozilla Preference reference - MDN
• Mozilla Networking Preferences - MDN

(For a comparison view of table and actual about:config settings, use the Fox Splitter (formerly Split Browser) extension or the new version of Fox-Splitter to show one above the other. Also, finding item documentation for a preference from within about:config can be sped up with the Prefsearch extension, which adds a context menu item to search Google for the selected preference item.)

Note: There is no indication in the table as to which items will take effect immediately, with a new window, or require restarting.

Spaces between the dots in preference names on this page exist purely for word-wrapping. They are actually not present and should not be thought as so. Editors of this page must take care to follow this convention in order to not force users to horizontally scroll this article.

Accessibility.

accessibility.* allows you to modify Find As You Type behavior, change how the [Tab] key works in web pages and XUL, turn on caret browsing, and tweak other accessibility-related features.

accessibility.* has been migrated to Category:Preferences.

Advanced.

advanced contains two prefs: Advanced.mailftp and Advanced.system.supportDDEExec.

advanced.* has been migrated to Category:Preferences.

Alerts.

alerts. contains three prefs, all related to the GUI: Alerts.slideIncrement, Alerts.slideIncrementTime, and Alerts.totalOpenTime.

alerts.* has been migrated to Category:Preferences.

App.

app.* allows you to modify application update behaviour.

app.* has been migrated to Category:Preferences.

Bidi.

Browser.

Config.

Content.

content.* allows you to configure the frequency of certain events when parsing and rendering web pages.

content.* has been migrated to Category:Preferences.

DOM.

Editor.

These settings affect libeditor (i.e. Composer and Mail). Most have no effect in Firefox.

Extensions.

This branch of preferences is usually used to hold settings for installed extensions, but there are a few that deal with extensions and themes in general.

Font.

The majority of the font.* entries are of the form:

font.(font property).(font type).(language code)

The (font property) indicates that the value of the entry is one of the following:

name-list 

A comma-and-space-delimited list of font names corresponding to the (font type) and (language code) (String)

name 

A font name (String)

size 

The font size, in pixels (Integer)

The (font type) indicates which CSS generic-family is being referenced in the value, or simply whether the font is fixed-width or variable-width. The (font type) is one of the following:

serif 

(e.g. Times)

sans-serif 

(e.g. Helvetica)

cursive 

(e.g. Zapf-Chancery)

fantasy 

(e.g. Western)

monospace 

(e.g. Courier)

fixed 

Any fixed-width font

variable 

Any variable-width font

The (language code) is an ISO 639-2 value.

Other font.* entries are listed below.

Geo.

General.

Intl.

Javascript.

javascript.* has been migrated to Category:Preferences.

Layout.

layout.* allows you to modify selection behaviour, control spell checking in textareas, modify the appearance of unstyled XML, and change the way pages are layed out.

layout.* has been migrated to Category:Preferences.

Mail.

See Mail and news settings.

Mailnews.

See Mail and news settings.

Middlemouse.

middlemouse.* has been migrated to Category:Preferences.

Mousewheel.

mousewheel.* settings:

These fall into two groups: vertical and horizontal scrolling. The general form is:

• mousewheel.(modifier key).(type)
• mousewheel.horizscroll.(modifier key).(type)

Modifier key is one of: withaltkey, withcontrolkey, withmetakey, withnokey, or withshiftkey

Type is then one of:

action 

integer value that determines the type of action:

• 0 - Scroll document by a number of lines (given by the numlines property)
• 1 - Scroll document by one page
• 2 - Move back/forward in history
• 3 - Make text larger/smaller
• 4 - Scroll document by a number of pixels (given by the numlines property)

numlines 

number of lines to scroll by if action is set to value 0, or pixels if action is set to value 4; otherwise, this value is ignored.

sysnumlines 

use system preferences to determine how many lines to scroll by

For example, to change (vertical) mousewheel scrolling to one page at a time -- set mousewheel.withnokey.action to 1

• a number of prefs similar to those described above, but with different "modifier" names, and horizontal and vertical prefs grouped under the modifier, as follows:
  • Modifiers:
    • .default.
      • with no modifier, or with more than one, the default settings are used
    • .with_alt.
    • .with_control.
    • .with_meta. (Command key on Mac)
    • .with_shift.
    • .with_win.
  • Types: (defaults are given in the above order of modifiers)
    • action (integer) (for vertical mouse direction) (defaults: 1 2 3 1 1 1)
      • 0: No action
      • 1: Scroll contents
      • 2: Back/Forward in history
      • 3: Zoom in/out
    • action.override_x (integer) (for horizontal mouse direction) (defaults: all -1)
      • -1: Use same setting as for vertical scrolling
      • 0 to 3: Use this setting, same meanings as above
    • delta_multiplier_x (integer) (defaults: all 100)
      • Value (in percent) multiplied by the delta value. Direction reversed if negative. Absolute value must be 100% or larger.
    • delta_multiplier_y (integer) (defaults: all 100)
    • delta_multiplier_z (integer) (defaults: all 100)
  • In particular, it is no more possible to scroll by whole pages independently of the browser content height. To make shift-wheel scroll by 5 times as much as with no modifier, set mousewheel.­with_shift.­delta_multiplier_y to the value 500 (meaning 500%).
• the following "individual" prefs:
  • mousewheel.acceleration.factor (integer, default 10)
    • factor to be multiplied for constant acceleration
  • mousewheel.acceleration.start (integer, default -1)
    • prefs for app level mouse wheel scrolling acceleration, number of mousewheel clicks when acceleration starts; acceleration can be turned off if pref is set to -1
  • mousewheel.min_line_scroll_amount (integer, default 5)
    • If line height is smaller than this value (in device pixels), each single scroll scrolls this height
  • mousewheel.system_scroll_override_on_root_content.enabled (boolean, default false)
  • mousewheel.system_scroll_override_on_root_content.horizontal.factor (integer, default 200)
  • mousewheel.system_scroll_override_on_root_content.vertical.factor (integer, default 200)
    • Prefs for override the system mouse wheel scrolling speed on the root content of the web pages. When mousewheel.­system_scroll_override_on_root_content.­enabled is true and the system scrolling speed isn't customized by the user, the root scrolling speed will be multiplied by these factors (expressed in hundredths, i.e. 200 means 2.00).
      NOTE: Even if mousewheel.system_scroll_override_on_root_content.enabled is true, the override isn't executed if Gecko detects that the user customized the system scrolling speed settings.
  • mousewheel.transaction.ignoremovedelay (integer, default 100)
    • mousewheel scroll transaction is held even if the mouse cursor is moved
  • mousewheel.transaction.timeout (integer, default 1500)
    • mousewheel scroll transaction period of time (milliseconds)

Network.

News.

See Mail and news settings.

NGLayout.

NGLayout stands for Next Generation Layout Engine. These preferences are largely low-level tweaks and debug options for how content appears in the browser.

Places.

The Places module manages bookmarks and history management system through sqlite. See also Places Expiration.

By default, Firefox calculates an appropriate maximum number of pages to store in the history based on the systems's available memory (etc.); history is not based on a length of time.

Plugin.

Print.

Privacy.

Profile.

Security.

Signon.

Snav.

These preferences are associated with Spatial Navigation, a method of moving focus on a web page with respect to the visual layout of elements. Spatial navigation is not present (compiled in) by default, so these preferences will only work in binaries compiled with ac_add_options --enable-extensions=spatialnavigation.

Toolkit.

UI.

Most of these preferences control settings that the browser attempts to read from the host operating system (e.g., your current color scheme). However, these can be overridden by manually setting the preferences to something else. For a more complete listing of the UI.* preferences, see UI Widget Look and Feel.

Update. and Update_notifications.

View_source.

XPInstall.

Miscellaneous preferences

See Mail and news settings for other miscellaneous preferences related to Thunderbird and Mozilla Suite/SeaMonkey mail and news.