Download Link: https://addons.mozilla.org/firefox/addon/globalchromecss/
This provides ability to apply custom CSS file for all users globally. This is mainly designed for corporate-use.
How to use
- Create a CSS file same to userChrome.css.
- Put the file as %AppDir%/chrome/globalChrome.css. On Windows, it is: C:\Program Files (x86)\Mozilla Firefox\chrome\globalChrome.css
- Install this addon globally.