I don't know anything about how programs choose between audio cards. How do you do that in other applications? ''Regarding audio output devices:'' If this page gives you an output device selection feature in other browsers, you could try the following change which lets pages see different audio "sinks" and might enable that same feature in Firefox. I don't know why it's disabled by default; it could be buggy: (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. (2) In the search box in the page, type or paste '''sinkid''' and pause while the list is filtered (3) Double-click the '''media.setsinkid.enabled''' preference to switch the value from false to true