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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/SeaMonkey, type about:config in the Location Bar (address bar) and press Enter to display the list of preferences, as shown below in Firefox 3. (If you see a page with the warning message, This might void your warranty!, click the button labeled "I'll be careful, I promise!", to continue.)
  • 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. In the context menu, select New then select the type of preference you are adding.

  • String is any sequence of text
  • Integer is a number
  • Boolean is true-false

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. (For more information about resetting preferences, see this article).

See also

External links