The speed increase in Firefox has been something I haven't seen in a browser update for years.
If you visit any page made in 2005 with any recent browser it will be unbelievably fast. Pages became more bloated in the last decade because they could.
chrome's day-one multiprocess support was really the big shift since it allowed js bloat to not kill your session, which in turn allowed more js bloat to exist, which in turn made ff slower..
But, like Firefox, they now have a big codebase to maintain and it's not so easy to get massive perf improvements because changing up architecture is painful.