ColorfulTabs wouldn’t work at all until this Firefox bug is fixed: https://bugzilla.mozilla.org/show_bug.cgi?id=1320585 And this bug hasn't been fixed ever since it was reported.

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