It's a nice idea but it only works on whatever window activating it via 'Add-ons' is on - i.e. it doesn't show counts on multiple FireFox windows.

While it could be useful in subliminally helping my brain find tabs by number/location and how many tabs there are, it's not really helping because my displayed tab/font size and tst-tabcount's orange/black colour scheme doesn't make it at all easy to see in my preferred 4K screen size, the count display in each tab also overlays (obscures) TST's tab count for collapsed trees, the two types of count would be useful to see side-by-side.

tst-tabcount could be really useful if only it were configurable for colours, size and position, and also supported multi-window counts.

Will have to deactivate for now as it's not helping me much as is, sorry, hopefully it will be improved in time.