Most of the preferences in Firefox can be accessed by using the interface available through
about:config. Any preference that is changed away from its default value is saved by Firefox to a file named
prefs.js in its Profiles directory. This is a file that is written by Firefox and should not be modified by the user. The user can provide Firefox his own settings in a
user.js, also placed in the Profiles directory. The
user.js preferences will override that in
prefs.js or the default.
Tried with: Firefox 4