From MozillaZine Knowledge Base
Note: This file only affects your Mozilla application's display (when you are using a particular profile). It does not actually change any web page or e-mail message. For example, if you use the file to change colors in e-mail messages, it does not affect messages that you send—so people who receive messages from you will see them using their own choice of colors, not using your choice of colors.
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 userContent.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 sample styles, see:
Moving and copying
You can move or copy userContent.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 userContent.css to get rid of any user styles defined there. To make the change take effect, you must restart the application.
Related files and folders
For some information about common problems, see: Editing configuration – Troubleshooting