I am using the latest Firefox 65 on macOS 10.13.6. Issue: No audio playback can be heard through the browser. I am using a Soundcraft Signature 22 MTK multi-channel interface where audio playback is routed through channels 21/22 through Audio Midi settings in macOS. It seems that Firefox is hard coded (and incorrectly assumes) all systems will default to channels 1-2 for audio playback. I can tell it's defaulting to 1-2 because if I route audio to those channels in macOS and then us the USB return on those channels on my console, I can hear sound. It should be respecting how audio is routed at the system level.