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.
For more info go here and here.
Tried with: Firefox 4