I hope that about:performance has not been nerfed still further. Currently I have an issue wherein some tab, unknown to me, begins using one or more cores at full speed. About:performance allows me only to search by the eco-friendly "energy" usage bit, which somehow does not reveal which tab is doing it. I kill off the most active tabs according to that stat, nothing happens.

And so I have had to default back to killing off a process and guessing which tab has crashed.