I haven't had a situation where I "can't do anything on other pages", but when I have tons (hundreds) of tabs open, Firefox regularly consumes 30% of my CPU time doing nothing. I have flash blocked and animations set to not loop, so you can't blame those.
And don't tell me I should have fewer tabs open. I should be able to use the browser however I want. Sure hundreds of tabs is not the normal use case, but I know there are others out there that do the same thing. Provide a better bookmark system and maybe I won't need so may tabs.
Just today, Firefox "forgot" all my bookmarks, so I had to delete my places.sqlite file to get them back. When I went to delete it, it was 124MB! No wonder the awesome bar was always so laggy. It is much faster now, but all my history is gone (not that I ever really use it, but I like to keep it). At least I got my bookmarks back.
I stick with Firefox, though, because I like it, I like the extensions, and nothing else I've tried has been a good enough replacement. Google Chrome can't handle the number of bookmarks I have, or the number of tabs I like to keep open (at least on my mac, I haven't tried the Windows version). Opera lacks extensions (though it's getting them in version 11) and is still a niche browser. Maybe it's time to try Safari again now that version 5 has extensions, but I still don't like that it is somewhat limited, like most Apple products*.
*Yes, I use a Mac, but that's because their OS is really solid and works well. It doesn't have audio dropouts like my Windows 7 PC, for example, and it doesn't have most of the hardware problems I've had with Linux (mostly video driver problems) and more commercial software works on Mac than Linux. I don't like the direction Apple seems to be going, making everything more closed, but I will stick with it until something better comes along or until I'm forced to switch.