> It's worth stating that things not working isn't the fault of Firefox. It's developers not testing their web apps with Firefox, since it has such a small market share.

Um. As a developer who frequently switches between them, it's both. There are animation issues on some of my own stuff that are specific to FF and that I haven't been able to pin down yet. (It's low priority because I'm considering reworking the whole frontend in something other than React anyway and I haven't ruled out this being some state or ref related bug.)

The whole point of initiatives like Interop has been to make browsers' rendering and other functionality more consistent.