As a daily Firefox on Android user, not catching up on standards is what hurts the most. Most painfully to me, all the WebGL stuff like [1] and some minor annoyances like [2]. Still, having uBlock origin among other extensions is a killer feature.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1884282

[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1897707