About:config

about:config is a feature of Mozilla applications which lists application settings (known as preferences) that are read from the profile files prefs.js and user.js, and from application defaults. Many of these preferences are not present in the Options or Preferences dialog. Using about:config is one of several methods of modifying preferences and adding other "hidden" ones.

Warning: Modifying preferences can, in rare circumstances, break Firefox, Thunderbird or the Mozilla Suite, or can cause strange behavior. Only do so if you know what you are doing or are following trustworthy advice.

Opening about:config

• In Firefox and Mozilla Suite, type "about:config" (without quotes) in the Location Bar and press Enter.
• In Thunderbird 1.5 or later, about:config is accessed via "Tools -> Options -> Advanced -> General -> Config Editor (button)". For earlier Thunderbird versions, use the AboutConfig extension.
• For Nvu, there's a compatible version of the AboutConfig extension here.

Adding, modifying, and resetting preferences

To add a new preference, context-click (right-click) anywhere in the list of preferences, select New then select the type of preference. String is textual, Boolean is true-false and Integer is numerical.

To modify an existing preference, context-click (right-click) on the preference, select Modify and type in the new value. In some cases, Toggle will be the selection offered for boolean (true-false) preferences.

To reset a preference to its default value or to remove an added preference, context-click (right-click) on the preference and select Reset. If you added the entry via about:config, the preference will no longer be listed after restarting the program.

See also

• The article about:config entries contains a list of valid about:config entries and their possible values. The article is a reference for what each preference does; it is not the place to actually edit your preferences! (See the section Opening about:config above.)
• Editing configuration
• About protocol links — other about: pages.

External links

• A brief guide to Mozilla preferences
• MozillaZine Tutorial for about:config — Step by step guide with screenshots.
• Mozilla Help: Editing Configuration files — user site: ilias.ca
• Preferential extension — An extension similar to about:config with extra features, such as documentation for some preferences.