From MozillaZine Knowledge Base
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.
This article is a companion article for about:config entries where the most important about:config variables are described.
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.
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.
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).
You can use the "Search" bar at the top of the about:config page ("Filter" bar in older versions) to filter the preferences that you want to inspect. The search bar is case-insensitive, unlike the actual configuration variables.
Some examples to filter results:
Some examples of regular expressions (RegExp) to filter results