Simple. I use either this one or Tab Count, they do the same. If I could know which one is the lightest, I would use.

Не знаю, зачем, но это работает

Please show number of active/unloaded tabs ! thanks

This is one of the few Firefox add-ons that help me keep me sane. It does exactly one thing, and stays out of your way, but is always there in a corner whenever you need it.

Не видно. Linux, i3wm.

Не показывает четырёхзначные номера страниц

It let me know which Firefox window contains the highest number of unclosed tabs that should be closed last in order to preserve my unfinished tasks. However, there's a three-digit limit.

Works but: Only shows 3 characters. The 4th 1,000+ is cut off on the right side. (though 1000 is too many for Firefox anyway). Customization settings would be good improvement.

Good extension, but font size is not customizable and could be bigger.

Works fine. Though with a large number of open tabs/windows the figures shown are truncated. Would be nice if that could be fixed.
Or maybe make the pitch of the figures customizable to fit everything in.
Hovering over the button shows the full numbers in the pop-up, so this is the work-around for the full info.

works great, couldn't find it at first (it showed up to the left of my minimize button) but it works perfectly and is simplistic. exactly what i need. you can look at the screenshots on the extensions page for examples. (I shoulda done that first)

great UI design. but slows down the entire browser:
https://github.com/DaAwesomeP/tab-counter/issues/33

Great. Easy to see open tabs numbers.

I found out why my laptop was running slow when I opened my web browser :) i had many open tabs that i opened long ago

love it! I just wish it could handle more than 3 digits, it gets hard to see the count once I have 1000 tabs open