Firefox has shipped Quantum in mainline for years. It may be on Chrome's level (I never tested), but regardless, it's nowhere near as performant as Safari.

Try running a very compute intensive script (that chokes your CPU/GPU). In my experience Firefox becomes unusable, Safari is still buttery smooth.

Now maybe Safari simply performs better, or maybe that's some OS level prioritization unfairly advantaging Safari, but to me as an end user, that's irrelevant.