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Editing Configuration Files

about:config

Many of the tips and tricks mentioned on this site require you to use about:config. To access about:config, type about:config in the Firefox address bar, and press Enter. This will display a list of used preferences, as well as a search bar at the top.

Double-click on a preference to change its value. To do add a new preference, open the context menu (right-click) on any existing preference, go to the New menu, and select the type of preference.

Some preferences may require that you restart Firefox in order to take effect.

Configuration Files

Some tips and tricks require that you edit configuration files manually. There are three files you should know about:

userChrome.css: Used to change the appearance of the browser.
userContent.css: Used to change the appearance of web pages.
user.js: Used to change various preferences.

All these files are plain text files you need to create in your profile folder and can be edited using a standard text editor, such as Notepad or Texturizer on Windows and gedit or kate on Linux.

Locate your profile folder

Before the configuration files are presented, you should know how to find your profile folder, which is where Firefox saves all your settings on your hard drive.

On Windows Vista/XP/2000, the path is usually %AppData%\Mozilla\Firefox\Profiles\xxxxxxxx.default\, where xxxxxxxx is a random string of 8 characters. Just browse to C:\Documents and Settings\[User Name]\Application Data\Mozilla\Firefox\Profiles\ on Windows XP/2000 or C:\users\[User Name]\AppData\Roaming\Mozilla\Firefox\Profiles\ on Windows Vista, and the rest should be obvious.
On Windows 95/98/Me, the path is usually C:\WINDOWS\Application Data\Mozilla\Firefox\Profiles\xxxxxxxx.default\
On Linux, the path is usually ~/.mozilla/firefox/xxxxxxxx.default/
On Mac OS X, the path is usually ~/Library/Application Support/Firefox/Profiles/xxxxxxxx.default/

%AppData% is a shorthand for the Application Data path on Windows 2000/XP/Vista. To use it, click Start > Run... (use the search box on Vista), enter %AppData% and press Enter. You will be taken to the "real" folder, which is normally C:\Documents and Settings\[User Name]\Application Data on Windows XP/2000, C:\users\[User Name]\AppData\Roaming on Windows Vista.

Firefox is capable of handling more than one user and thus, more than one profile. The path examples above refers to the default profile that is automatically created when you start Firefox for the first time. You can manage any number of profiles by using the Profile Manager.

userChrome.css

This file sets the display rules for various elements in the Firefox user interface and is located in the sub-folder called chrome in your profile folder. This file does not exist by default, so you need to create it before you can start adding your preferences. There's actually an example file that exists by default, called userChrome-example.css. Basically, you can just rename that file by removing the -example part.

userContent.css

This file sets the display rules for web content and is located in the sub-folder called chrome in your profile folder. As with userChrome.css, this file does not exist by default, so you need to create it before you can start adding your preferences. As with userChrome.css, there is an example file that exists by default, called userContent-example.css. Basically, you can just rename that file by removing the -example part.

user.js

This is the main preferences file for Firefox and is located in you profile folder. The file does not exist by default, so you need to create it before you can start adding your preferences. In almost all cases, edits made using the user.js can be done via about:config, which is the recommended method.

Windows users: If you know how to create a file, just skip this section. Before you can create the file, you must make sure that you can see the file type extensions in Windows. Start Explorer (just click on the My Computer icon on your desktop) and select Tools > Folder Options.... Click the View tab and uncheck the Hide extensions for known file types option. Now, go to your profile folder and select File > New > Text Document. The default name New Text Document.txt should be displayed. Change the name to user.js (click Yes on the file type warning that appears).

Linux users: I assume that you know how to create a file if you're running Linux. Just create user.js in your profile folder.