I'm pretty sure this used to be the 'right way' of doing things: use 'px' when the text shouldn't be resizable (apart from via page zoom) and 'em' when it should be resizable.

I speculate that the other browsers opted to make 'px' text resizable because a large number of sites use 'px' text, perhaps inadvertently due to lack of accessibility testing. So for these sites it's more user-friendly to make 'px' text resizable, than forcing the user to zoom the whole page.