From MozillaZine Knowledge Base
userChrome.css in the chrome folder is a CSS file that can be used to change the way Mozilla applications' interfaces look. This file does not exist in a new profile. You can create it manually. If you just want to move/remove/rename menus and menu items or change/disable shortcuts use the Menu Wizard add-on instead.
To create this file, use your operating system tools.
Go to your profile folder. Then go to the chrome folder there, creating it if necessary. In the chrome folder, create a plain text file named userChrome.css.
This file is designed to be manually edited using a text editor. Anything valid in a CSS file is valid in this file. To make the changes take effect, you must restart the application.
For Mini Tutorial (old) see Section 4 here -
For sample styles, see:
Moving and copying
You can move or copy userChrome.css to a different profile. If it imports or references other files such as images, then you must also move or copy these other files. To make the changes take effect, you must restart the application.
You can delete userChrome.css to get rid of any user styles defined there. To make the change take effect, you must restart the application.
For some information about common problems, see: Editing configuration – Troubleshooting
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