Fast Rendering and minimal resources usage post e10s, Quantum and Servo components like WebRender. While a lot of these landed a long time ago they then took years to further optimise and iron out edge cases. Chrome used to be the fastest while Firefox used to be slower but memory efficient, both are no longer true as both Chrome and Firefox has improved in those respective areas. Compatibilities are perhaps at all time low? Even during Firefox winning IE era it wasn't anywhere as good as today in terms of compatibility.

Have you used it recently? It's as fast as any competing browser, has no showstopping problems, has native vertical tab support and is the only place you can still get ublock origin.

AI!