In practice I've found that people claiming site X works in chrome but not in Firefox is usually incorrect, and unless you actually try it you won't know. I use Firefox and have done for 15+ years, and I genuinely can't remember the last time I had to switch browser to use a page or webapp.

Yes, but some of us are forced to. As long as Mozilla claims that implementing WebMIDI is impossible that's how it will remain. It's technically in there somewhere but it just doesn't work or requires all kinds of unstable trickery, not good enough for a normal user.