Editing Firefox Configuration

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

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.

Information from mozilla.org