This looks interesting and I'm guessing fairly new as an add-on. The first thing I notice is that the Window1/Window2 order shown is not what I consider to be my order or Firefox windows, i.e. my main window to me is '1' but this add-on shows it as '2' - perhaps if there were some way to define which window should be which in the count.

The other thing I notice is that the font display and arrangement feels a little crude and large for my compact liking - it's using a mono-space font on multiple lines which I appreciate helps with the layout of addition figures but maybe it would be nicer to just have the data display on one or more lines in a more compact way (it doesn't matter if each window's tab counts are under each other or not), just having something like this would do:

W1: 2/56, W2: 23/543, W3: 1/1, T: 27/600

Then maybe it would all fit on one line on my tab display and if not, in a small font/size on two lines (with narrow line-spacing) it could be informative without taking up valuable vertical tab space, i.e. treat it like a status line of data rather than an addition sum, one window on top of another.

The above figures are my count right now, W2 is my main window, W3 is that of my password manager and it would perhaps also be useful to exclude that maybe.

Otherwise, a nice idea and I look forward to it evolving.

Oh, one last thing, it seems to have obscured or obliterated the 'New Tab' description even though the function remains if I click in that area used for the tab count display.