On ff4, the ram usage just keeps going up whilst its idle, it keeps going up until you do something like close the active tab or load a new page in that tab, at which point the ram will go back down again (not necessarily all of it but most of it).
I don't get that. It would be a blocking bug too, but it's not on bugzilla. I'd imagine it's caused by a plugin or addon. The fact that it is in the javascript heap would point to that too.
there is at least 10 reports on bugzilla regarding memory leaks in ff4, my guess is there is actually at least triple that but 10 is what I found with a quick search. 2 of them are mine.
also on ff4 if its left idle for too long it eventually crashes with the memory usage, mine last crashed when idle for 14 days and 7.6gig of ram usage.