the good: you can customize color and it has options for resizing. Overall it was pretty decent although still not quite what I was looking for. Definitely like that it *has* preferences and being 'experimental' I wasn't sure what to expect coming in.

the bad: the resizing options didn't work for me at all.

the ugly: it had multiple different tab counts (current/open/hidden). I wanted to just have a plain 'all open tabs count' as the *only* number displayed but I was only able to turn off the 'hidden' tabs count in the settings. Due to this and the resizing not working correctly for me, this would display as "abc / xyz" with the 'a' and 'z' positions getting cut off when I had a large number of tabs open. Since I couldn't fix the sizing and I couldn't change it to only show *one* number, unfortunately I must move on.

Best of luck.

For those interested in similar criteria, so far Tab Count Icon by lllusion3418 has most of what I was looking for above but I have issues resizing in that one too.. maybe it is a limitation of WebExtensions / Linux / the combination?

Edit: FF v89.0.2 on Fedora 33 Cinnamon