Sometimes the computer fan starts spinning like the computer is going to take off, and when I look at the Activity Monitor I only see that "Firefox" is the culprit process. Then turns out that I have a ton of open tabs and I cannot afford to guess which one of them all is misbehaving. It would be super useful to be able to exactly pinpoint how much memory / cpu power is each tab consuming. Filing under "Profiler" in a vague guess of where this should go, but feel to re-file under some other, more appropriate "department"!

Yes, please let's get this! :) But, I am pretty sure this is a dupe of bug 400120 or bug 515352. I'll pick bug 400120.