has some suggestions for what to put in there, aimed at achieving pre-FF-98 look (among other things). In that directory, create a directory called chrome, and in it create file userChrome.css. Open the profile folder: in about:support, find the row called Profile Folder and click Open Folder next to it. Check this for detailed instructions, but in short:
It can change the tab colors, but I didn't have to use it for this purpose: I have FF theme set to 'System' and the system theme I'm using paints the inactive tabs (and the rest of the top bar) black even after the update to FF 89.īoth can be fixed.
A more long-term solution is to use userChrome.css. The option to disable the new UI will inevitably be removed from about:config.