What type of issue is this?

Browser bug (a bug with a feature that may impact site compatibility)

What information was incorrect, unhelpful, or incomplete?

Firefox is listed as "Full support" but unlike Chrome it does not fire this event when a new MIDI device is plugged in.
Also, a page refresh does not work. The new port is only available after a browser restart.

What browsers does this problem apply to, if applicable?

Firefox

What did you expect to see?

This shortcoming should be mentioned, rather than "Full Support". Maybe even on the main MIDIAccess page, since requiring a browser restart is a pretty severe problem, IMHO.

Did you test this? If so, how?

A WebMIDI test did not show any devices after I plugged in my device. It did work in Chrome. I had to restart Firefox to make it work, and then I discovered the bug report that there is no hot-plug support for MIDI in Firefox, it only scans once on browser start.

Can you link to any release notes, bugs, pull requests, or MDN pages related to this?

https://bugzilla.mozilla.org/show_bug.cgi?id=1802149

Do you have anything more you want to share?

No response

MDN URL

https://developer.mozilla.org/en-US/docs/Web/API/MIDIAccess/statechange_event

MDN metadata