Works well for me (FF 63.0.1 32 bits), simple, no options, sits next to the URL bar. The badge is blue/green, with black letters, discretion ok. On some site, like on this one, sometimes take a little delay to update page number, but short.

I always wondered how many tabs my main Firefox window had, and now I know: 535 tabs. I knew there were a lot, but maybe I should clean up. :-D And I have 6 more FF windows...

For 5 stars, make the icon respond to clicks and list all open windows with the count of tabs next to them and possibly a total count beneath. Clicking a window in this menu should bring forward the clicked window (if possible).

Needs to be configurable like Tab Counter 1.9.9.3.