ColorfulTabs wouldn’t work at all until this Firefox bug is fixed: https://bugzilla.mozilla.org/show_bug.cgi?id=1320585

I’m lucky we got it to work at all. Currently we can only color the current tab. Thanks to Patrick for coming up with an innovative idea. Please also see: https://www.addongenie.com/forums/topic/colorfultabs-disabled-in-firefox-quantum. Just use a previous version on Firefox ESR and disable automatic updates (at your own risk).