I have an external multi channel sound card (MOTU Ultralight Mk3 Hybrid). Gut feel is the hardware/driver is not relevant. I see there are multiple old bugs that have been closed. As such I have been told to open a new issue. M1, Mac OS 13.4.1 - I am an experienced audio engineer so I know my way around multi channel audio config on Mac. Audio was working fine from Firefox with a default setup. I had a need to alter some system sound settings for multi channel audio, unrelated to Firefox. When I reverted those settings. Firefox was sending audio to the non default audio channels on my sound card. Safari, Chrome, Mac OS and Spotify were obeying the reverted config and sending the audio out the main outs. Only Firefox, for some reason was sending audio to channel 3&4. I could tell because I was getting a level on those channels. I have tried the following: * Rebooted * "refreshed" Firefox * Used AppCleaner to completely remove all Firefox and my sound card apps, drivers and preferences * Cleared cache * Triple checked all my factory config * Retested Chrome, Safari etc... all good with audio coming out the main out stereo channel Firefox still sending audio to 1 & 2 (3 & 4 as the crow flies, and as the system sees them) since 1 & 2 are actually "Main out" The audio is being routed. I just can't for the life of me figure out why Firefox is selecting the wrong channels. It used to work, and now it's stuck this way.