The FUD is that the # of tabs has anything to do with it. Also, these problems have been getting a _lot_ better with recent releases, which are much better at avoiding blocking the UI thread. It's not perfect, but it's not something you see very often either. Let me put it this way: I can't even remember the last time I've had a UI slowdown, and I use FF on various machine with lots of tabs all the time. (Firebug's still really slow, though).

So when you say "some heavy JS usage" what exactly do you mean? Certainly not stuff like google docs/mail/calendar, and they're all heavy on the JS...