From MozillaZine Knowledge Base
A user.js file is an alternative method of modifying preferences, recommended for advanced users only. Unless you need a user.js file for a specific purpose you should use about:config instead. The user.js file does not exist by default.
Important: Once an entry for a preference setting exists in the user.js file, any change you make to that setting in the options and preference dialogs or via about:config will be lost when you restart your Mozilla application because the user.js entry will override it.
About the user.js file
A user.js file can make certain preference settings more or less "permanent" in a specific profile, since you'll have to first delete or edit the user.js file to remove the entries before the preferences can be changed in the application. This has the advantage of locking in certain preference settings. A user.js file is also a way of documenting preference customizations and it makes it easier to transfer customized settings to another profile.
When you launch your Mozilla application, valid preferences you've added to the user.js file are automatically copied to the prefs.js file (located in the same profile folder) where all user-set preferences are stored. For this reason, you should make a backup copy of the prefs.js file before you create or edit the user.js file.
Creating the user.js file
To create a user.js file, open a text editor such as Notepad and save the empty file as "user.js" inside your profile folder. (If you use a Windows text editor, make sure you unhide extensions for known filetypes in Folder Options, so that the file isn't really called "user.js.txt". See also the manual editing advice.)
Adding user.js entries
To add preferences to user.js, open the file in a text editor such as Notepad, and type in the preference entries (and any comments) as follows. (After adding the entries, you must save the file! Changes will not take effect until you restart your Mozilla application.)
Valid preferences are entered via the user_pref command, as demonstrated below.
// Don't show bookmark icons user_pref("browser.chrome.site_icons", false);
Many Knowledge Base articles discuss individual preferences and their values. For a comprehensive list of valid preferences and their values, consult the about:config entries article.
Note that in Windows, if the preference value is a folder path then the path separator must be two backslashes. For example,
// Relocate parent directory for browser cache user_pref("browser.cache.disk.parent_directory", "D:\\Mozilla\\Firefox\\");
Editing user.js entries
To edit the user.js file, open the file in a text editor such as Notepad, make your changes, then close the file and save the changes. Note that removing or "commenting out" a preference line from the user.js file will not remove changes written to the prefs.js file (see below).
Removing user.js entries
To undo preferences that have been set in the user.js file: