Security concerns that Chrome seems to be able to deal with just fine. If that's the issue then block sysex but allow the rest, the midi (v1) protocol isn't that hard.
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).
That is misleading. Mozilla chose not to implement it for security concerns. More context: https://github.com/mozilla/standards-positions/issues/58#iss...