I use Linux as well (several distros), but I haven't messed with it much lately and have been developing Windows utilities so it's been a while, but you may find good information here:
Ubuntu - Userchrome css tweaks for firefox quantum and later
It seams you might have to add:
@namespace xul url(https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
to the top of your userChrome.css file
