Pros:
* Works and displays tab count
* Has Preferences where you can change the background
* Single, unified tab count without wasting space displaying "in window / all tabs / etc"
* Has tooltip on hover

Cons:
* Took ~2-5 seconds longer to initialize than most of the other tab counters I've evaluated today (~10 so far).
* Only preference is background color. While this is good, many other tab counter addons also allow changing foreground color / type of tab that is counted.
* Tooltip doesn't really seem to provide any new info
* Font size seems small to me. YMMV

If all you want is a simple tab-counter, there are faster options.

If you want to be able to customize more, then I'd recommend checking out any of the following as alternatives: 'Tab Count Icon by lllusion3418', 'Tab Counter (Tilwa fork) by tqdv', or 'Tab Counter (fork) by vhfmag'

The biggest issue for me was that it was just too small on my screen (viewing from about 10ft away on a 50" TV set to 1920x1080).

FF v89.0.2 on Fedora 33 Cinnamon

It incorrectly show the number of open tabs of the active window on all windows, so you need to go to the specific window to see how many tabs that are open on it. So it doesn't work that well if you are using more then one window.