There's been a CPU profiler in the debugging tools for over a year, and before that there was one in the addon repository with documentation on the mozilla wiki.

Could be easier (like Chrome's task manager) but it's not as hard as you seem to think it is.

P.S. Chrome's task manager isn't accurate anymore because now a bunch of work is offloaded from the per-page processes to special catch-all processes like the 'GPU Process'. So this is actually a harder problem to solve than it seems at first glance...