The features that firefox does not support are few and far between, and, IME, usually things you
do not necessarily want supported.
As a user, I do not want nor need my browser to support AppleScript. AppleScript is something that should not exist. In somewhat typical apple fashion, it's some NIH platform-specific bullshit that nobody really cares about and is only half-assed supported even on it's native platform. The only way to deter Apple from creating these sisyphus-ian pieces of software is to just stop supporting them and force their hand to use something less bespoke. Although, Apple is not the only culprit of this - nor are they even the worst about it.
If I had my way, Mantle would not exist, iMessage would not exist, and some others. We would live in a perfect utopia and then we'd all hold hands and sing Kumbaya.