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-c