userChrome.css is a CSS file in the chrome folder within the Profile Folder.

It can be used to change almost every part of how Firefox looks.

It doesn’t exist by default, so if it doesn’t you need to create it by just making an empty text file called userChrome.css in the chrome folder in the Profile Folder.

Here is where you can find your Firefox Profile Folder:

Windows XP

Example:    C:\Documents and Settings\<user name>\Application Data\Mozilla\Firefox\Profiles\<profile folder>

Note:   The Application Data folder is a hidden folder; to show hidden folders, open Windows Explorer and choose “Tools → Folder Options → View (tab) → Show hidden files and folders”.

Windows Vista

Example:   C:\Users\<user name>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile folder>

The AppData folder is folder is a hidden folder; to show hidden folders, open a Windows Explorer window and choose “Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders”.