Finally Mozilla does something sane! This is really great news, and I could see this being a pretty big thing or even a major way of doing web enabled IoT.

I've been regularly checking that page, delighted with every step of progress as WebMIDI gets implemented in Firefox.

In about:config, I have dom.webmidi.enabled set to true. I'm using this test page and waiting for the day it starts working for regular version of Firefox.

https://versioduo.com/webmidi-test/

In Firefox 98.0, it actually works but is well-hidden. You not only have to turn on dom.webmidi.enabled in about:config as you've already done, but also need to manually add 'Access MIDI devices' and 'Access MIDI devices with SysEx support' permissions for the page in Tools -> Page Info before reloading, as otherwise it silently fails instead of prompting for confirmation. Hot-plugging interfaces after the browser is launched also doesn't seem to work yet.

Oh joy! It works - thanks for the tip about manually adding permissions to the page. I'll have fun with this.