It's the setSinkId function which let's you select an audio output (https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaEl...) but it looks like I was mistaken -- it should actually also work with Edge and Opera. Will update the message for clarity...
Also of note that Firefox's Web Audio API implementation just isn't very good in general. It's my daily driver, but I won't run Airsonic in it, because after half an hour or so the music reliably starts glitching. Fine in every other client, so it's definitely a Firefox thing, and iO-808 [1] also calls it out in an alert if you go there in Firefox. (And also starts glitching out after a while, if you decide to ignore the alert and take your chances.)
Granted, a glitchy audio implementation might be just the thing for this use case...
Its MDN doc (https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaEl...) shows it as a not-quite-finalized feature, but even so, seems odd to have a pref just for it.