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.