Tried Audible Genius again just now to check, but still isn't supported, although now that I look at the console it looks like this may have something to do wit

Tried Audible Genius again just now to check, but still isn't supported, although now that I look at the console it looks like this may have something to do with their Meteor and analytics setup, so not audio related. Tried Flowkey again now as well and although it required the Jazz-MIDI plugin (to permit the browser access to my MIDI controller), with that plugin I was able to use Flowkey in Firefox. So it looks like my concerns were outdated.