Provided this is anecdotal evidence, in my experience it's the opposite. I gained an enormous amount of performance-related comfort after switching to Linux Firefox from Windows Firefox/Chrome (where a JS-heavy web site that I use frequently would cause memory overflow and would stall the laptop). I've not experienced any problems with CPU usage since. I've not experienced the other problems either, though I don't use WebGL. You make it sound like Firefox is heavily broken for everyone. That's not the case.