Try using Firefox on the world's most popular streaming site Twitch.Tv and report back, let it sit there for a while and try for example use multiple streams to check different perspectives. It slows down to a halt.

Also I can't for example leave Firefox alive while playong some games or it starts to create stutter in the games.

I tried profiling but the Firefox profiler just hangs all the time or uses so many resources it becomes unstable ironically