So I've found that Firefox doesn't seem to detect my headset microphone if it's plugged in while Firefox is already running. Going into a Google Meet meeting, it asks me if I want to allow the use of my microphone, and only shows a device that was plugged in already (a Rift S headset microphone) - unplugging, replugging, and refreshing the page all have no effect. If I close down and restart Firefox completely with the headset plugged in, it works. After searching, apparently there was a similar bug that appeared around Firefox 36 (I think?) that was fixed soon after - In any case, it seems to have reared its head again.