When using web video conferencing services (Google Meet, Teams, etc.) that run in the browser, on a Windows system, Firefox only offers a single output device: the system-configured Default Device. It does not offer the system-configured Default Communication Device. (I can swap the default before a call, but that is cumbersome at the end of the day.) In this instance, the default is the speakers built into my screen, through HDMI. The communication default is my headset, which also provides the microphone (which is recognised and works fine). The issue is not experienced with Chrome (the only other tried). It has been an issue through many years, on multiple machines, with various default sudio out devices attached.