The Song Fight Bulletin Board

In Firefox > Preferences > General > Applications, I can set "MP3 Audio" to "Save file", or to use the Ubuntu default app (Audacious or VLC Media Player, depending how I've set the Ubuntu default), but no matter what I do, it opens in some nameless browser player with one thin line.

When I change the preference, handlers.json is updated, but the behaviour doesn't change.

Of course I've looked online for advice, but nobody else is asking about this. Does anyone have any ideas? Where would be a good place to ask?

That's the stand-alone media player jumping in and intercepting the link. You can turn this off for all media (audio/video) links: Go to the URL "about:config" and toggle the setting "media.play-stand-alone".
Or use the classic workaround: right-click MP3 links and choose "save target as".

Thank you, that worked like a charm.