As FF runs for a long time with many tabs, I get longer and longer pauses doing simple tasks -- scrolling, switching tabs, popping the right-context menu, setting focus into a field, etc. Sometimes, this sluggishness seems linked to (and fixable by closing) a few tabs of the most Javascript-heavy sites (those with many ad inserts, background status polling, etc.). I'd hoped FF4 would help but it's just about as bad as 3.X. Generally restarting, even with the exact same tab set, helps (for a while).

Separate but similar, sometimes I'm doing nothing and no tab is visibly busy but Firefox is reported as using 50-100% (out of 200%) CPU time. Give me any proxy for tab activity (object allocations, timer callbacks, method-dispatching, whatever) and I can probably kill the miscreants.