Note, this version disables userChrome.css in new profiles. If you need it, enable it in about:config: toolkit.legacyUserProfileCustomizations.stylesheets

Note, this version disables userChrome.css in new profiles. If you need it, enable it in about:config: toolkit.legacyUserProfileCustomizations.stylesheets = true I spent some time wondering why it's not working. The key name is totally not obvious, especially since it doesn't have anything resembling userChrome in it.