Before adding the userChrome.css, you must enable loading it in the Firefox config (Firefox 69+):
- In Firefox, browse to
about:config(accept the warning prompt) - search for
toolkit.legacyUserProfileCustomizations.stylesheets - double click the entry so it says
true
Here is how to find your Firefox profile folder:
- Windows Default:
%APPDATA%\Mozilla\Firefox\Profiles\xxxxxxxx.default\ - Ubuntu Default (apt):
~/.mozilla/firefox/xxxxxxxx.default/