Firefox has a suite of `about:` URLs[1], one of them is about:performance which will show you the Task Manager (their name) which has a sort of CPU and RAM column. It even shows you when it's javascript on a tab chewing up resources and not the extension.
I don't know if the situation has improved, but at least in past it seems about:performance has a problem picking out problems created by an addon doing a ton of DOM manipulation that chews up resources in the tab. Bisecting addons or my preference, a clean profile or safe mode, was usually what I suggested, but YMMV.