I tend to have a lot of tabs open in Firefox at any given time. It’s not unusual for me to have 20 or 30 open, and I’ve been known to hit 40 or 50 at times. (I wouldn’t survive without the sessionsaver extension.)
This can sometimes get Firefox pretty upset. Windows Task Manager shows nearly 1GB of Mem Usage. And sometimes CPU just hovers at or near 100%.
Sometimes Firefox becomes nearly unresponsive. This is a sure indication that it’s time to shut down and reboot, but it can take a long time just to get focus under these conditions. One thing I’ve found that works is, ironically, to lower the priority of the firefox.exe process in Task Manager. If you set it to BelowNormal, it will usually help Firefox to backburner what it’s doing (must be garbage collection or memory consolidation) and come back to processing UI events. But it really is a good idea to at least shut down Firefox, if not the whole machine, when you get to this point.
One not-so-funny thing about having a bunch of tabs saved in a session: I once opened Firefox while at some public place with broadband, maybe a coffeehouse or hotel, I don’t remember. I had thirty or forty tabs saved in my session, and when the blast of requests came from my machine all at once, the wireless router decided I was up to no good and blocked me from further Internet access. I’m surprised it only happened that one time.