Why? Hand waving and tantrums don't mean that what OP is experiencing is representative of the state of Firefox on OSX. OP is positing that it's this ancient bug that Mozilla developers have just ignored because they're so awful/incompetent/lazy/whatever, but there's no way it would be problematic on hardware that dates back to the original problems.
It just smacks of frustrated user doing something out of the norm and falling prey to this idea that their own experiences are OBVIOUSLY representative of the norm.
Me? I'd guess that OP is doing something like running with a non-scaled display, running something else that's disabling the integrated GPU, running a poorly behaved corporate plugin or anti-virus, loading something so memory intensive the computer is swapping to disk, has a poorly behaved corporate font installed (or too many fonts), or something else along those lines. Basically something that seems normal to OP but is, in reality, not.
I've seen plenty of poor behavior from Firefox and from the Mozilla devs, but this idea that Firefox just performs poorly across the board on OSX (or some increasingly specific subset of OSX) seems very unlikely given how much focus Mozilla has been giving to performance as of late. More likely I expect there's just a very vocal minority.