Regrettably it's not working on Firefox[1], but hey at least I can report wrongthink YouTube videos.
Note this 8(!) year old bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=836897
It's embarrassing.
That is misleading. Mozilla chose not to implement it for security concerns. More context: https://github.com/mozilla/standards-positions/issues/58#iss...
Meanwhile, there are 100's of WebMIDI capable applications now and Mozilla still claims there is no demand for a feature they do not support. That's backwards, of course a feature that you do not support is not in demand because people simply go elsewhere.
The cumulative effect of a lot of little crappy decisions like this is part of why FireFox' market share eroded (the other is Google's anti competitive behavior).