I'm using Ubuntu 20.04 and a focusrite scarlett solo external USB sound card.
The sound card is set as the audio output device in settings and the test works and, as far as I can tell, the rest of my system uses the USB device.
Firefox however is using the laptop in-built speakers. Strangely enough with certain things the USB sound card is used, e.g. the "error sound" produced when no matches are found after searching a webpage (Ctrl + F) but certainly youtube, vimeo and spotify are using the laptop speakers.
I haven't found any help online that's not quite outdated.