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User.js file

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The user.js file, located in the profile directory, is used to customize preference settings in Mozilla-based programs. Since the user.js file does not exist by default, you'll need to create the file before you can start adding your preferences. The valid preferences added to the user.js are copied into the prefs.js file (also located in the same profile directory) which then stores all the preferences for the current user. For that reason, before you create or edit the user.js file it is advisable to make a backup copy of the prefs.js file. Although the prefs.js file can be modified using (about:config), or, in some cases, through the options and preference settings within the Firefox/Thunderbird/Mozilla Suite program's user-interface, the preferences you add via the user.js file will never be modified unless you yourself edit the user.js file contents.

Creating the user.js file

If user.js does not exist in your profile directory, create it by opening Notepad or another text editor, type in the preference entries (and comments) then save the file as "user.js". (On Windows OS, make sure you UNhide extensions for known filetypes in Folder Options, so that the file isn't really called "user.js.txt".)

Valid preferences are done via the user_pref command, as demonstrated below. // denotes a comment, so anything after these two characters on a line is ignored. A valid preference entry always begins with user_pref and always ends with a semi-colon.

// IE Favorites (bookmarks) are imported the first time Mozilla is run.
// This preference is to determine whether they are shown in the bookmarks menu
// Default: true (Show IE Favorites under bookmarks)
user_pref("browser.bookmarks.import_system_favorites", false);

For a list of valid preferences, consult the about:config entries article.

Changing user.js entries

To edit user.js, open it within your text editor, make your changes, then close the file and save the changes.

To undo preferences that have been set in the user.js file:

  1. Remove the unwanted preference entries fom the user.js file as a first step. However, since the same preference is now written to the prefs.js, a second step is needed:
  2. Also remove the preference entries from prefs.js, either by direct editing of the prefs.js file (not recommended) or by resetting the preference via about:config.

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