Similar to what was done for chromium, this is a request to auto-connect the audio-record interface for the firefox snap (until recently the snap still plugged the pulseaudio interface, this was recently changed in favour of plugging audio-playback and audio-record).
It is a pretty common use-case to do video-conferencing or other webrtc-based applications within browsers these days, and without the audio-record interface the firefox snap will report that no microphone can be found, which will lead users to report bugs because they want this feature to work out of the box.
Besides, firefox prompts for permission to allow microphone capture on a per-domain basis, so that’s an additional level of security for actual access to the hardware.