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
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.