There is no default config file.<br /> Firefox stores the values of prefs that have been modified (i.e. show as user set) in the prefs.js file in the Firefox profile folder. You can use this button to go to the currently used Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox

If you want to reset a modified preference to its default value, right-click it and choose Reset. Reset preferences that normally don't exist (like those added by extensions) will be removed after restarting Firefox. * http://kb.mozillazine.org/About:config Firefox only keeps track of modified preferences in the ''prefs.js'' file in your profile folder. If you rename this file while Firefox is closed, you'd start over with the default set of preferences (though any add-ons you have installed would create preferences again once Firefox is restarted). It's also possible to set preferences at startup in ''user.js'', or to lock them for all users. * [http://kb.mozillazine.org/Profile_folder_-_Firefox#Files_and_folders_in_the_profile Profile folder - Firefox - Files and folders in the profile | MozillaZine Knowledge Base] * [http://kb.mozillazine.org/Locking_preferences Locking preferences | MozillaZine Knowledge Base] If you want to see what preferences look like when first starting Firefox, you can start Firefox in a new profile. * [[Use the Profile Manager to create and remove Firefox profiles]]