I don't even want process per tab, I'd be happy with something much simpler: Tell me which tab is causing Firefox to use 100% CPU! This happens fairly frequently with lots of tabs open (I guess I use a lot of websites with really bad javascript) and the only recourse is to either quit Firefox entirely and re-open it, or play a guessing game.

Firefox will stop any client side JavaScript after 5-6s and ask whether you want to keep running the slow running script or not. If will even change to that tab before prompting you. Isn't that what you just described?

Only if it totally locks up the browser though, not if the browser remains responsive but eats a lot of cycles.