MFH: r533208

www/firefox: add microphone note to pkg-message

Only PulseAudio appears to return a list of available /dev/dsp* for
cubeb consumers to allows users choose output/input at runtime.
Mozilla doesn't maintain other backends, so it's up to the community
to implement missing features.

https://searchfox.org/mozilla-central/search?q=.enumerate_devices&path=media/libcubeb

Approved by: ports-secteam blanket