A few big things and lots of small things.

Big performance wins recently optimizing some core operations:

https://bugzilla.mozilla.org/show_bug.cgi?id=1994067 https://bugzilla.mozilla.org/show_bug.cgi?id=1995626

which brings it near chrome performance without the new v8 optimizations.

Still more work to do, including avoiding extra copies just like v8, and exploring more simd etc. Generic slow items for toBase64 and fromBase64: https://bugzilla.mozilla.org/show_bug.cgi?id=2003299 https://bugzilla.mozilla.org/show_bug.cgi?id=2003305

extra copying of results: https://bugzilla.mozilla.org/show_bug.cgi?id=2003461 https://bugzilla.mozilla.org/show_bug.cgi?id=1996197

No reason all browsers would not be able to be similar in performance eventually. Pleased this was noticed and being worked on by both v8 and Firefox team

Mozilla's "privacy" image prevents them from knowing what their browser actually does in the wild, while Google collects CPU time profiles from user devices, comprehensively, and hammers down the hotspots they find, and that refinement has been going on for many years.