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.
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.
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/