"Many."
...792. I've used SessionSaver since it was new, I think back in the Firefox v3 or v4 days, before it was built-in.
It's a similar story at work, though each week I'll at least try to go through and prune out a few. Things get shuffled and priorities change quite often, so tabs for one project get left behind a lot, and are sometimes buried.
Now I just wait for the day, shortly before Half Life 3 comes out, when Firefox will properly support multicore processors like the other major browsers, assuming it lives long enough for that. :\
Cleaning up tabs is like cleaning up anything else: I probably don't do it as often as I should.
Over 700 sites open? How much RAM do you have on that computer? I have 8 GB and when I open 40 sites or so on Chrome, the computer slows down a lot and this is a month old desktop with i3 6th Gen 3.7 Ghz.
Firefox hits around
800MB (I think) 1.1GB once it finally opens. It doesn't make a separate process with lots of memory allocated for every single tab.
When/if it goes multicore, I hope that there'll be the ability to limit the number of CPU cores it can use. One major feature I, and probably lots of other people, like about multicore systems is that there's usually at least a core "spare" to let the computer remain usable, without having to go to Task Manager to change process priorities.