How many windows/tabs is a much more relevant question in my experience.

FF doesn't load old tabs so you can't just count them if they've been carried over since a restart.

true, active tabs matters, but people that have hundreds of tabs open but only rarely open new ones or touch old ones are probably also quite rare.

I open and close one or two times per day. Might have a dozen+ open at once, not a huge number.

I usually have 800 to 1000 tabs open and Firefox only uses a few GB.