About:config

about:config (one of the special about: links) lists application preferences read from "prefs.js", "user.js" and from program defaults, many of which are not present in the Options and Preferences dialog. You can use about:config to modify these preferences and to add other "hidden" preferences.

Warning: Changing these values can, in rare circumstances, break Mozilla Suite, Firefox, or Thunderbird or cause strange behavior. Only modify these preferences if you know what you are doing.

Opening about:config

• In Firefox and Mozilla Suite, type "about:config" (without quotes) in the Location Bar and press Enter.
• In Thunderbird, use the AboutConfig extension. Starting with Thunderbird 1.5, about:config is accessible from the "Tools" menu (Options > Advanced > Config Editor button).
• There's a version of AboutConfig extension compatible with Nvu 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

• 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 edit your preferences.

References

• 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.
• About protocol links — other about: pages.