I use an audio interface that has four outputs. The standard outputs are 1/2 (Stereo outs). All my system audio runs through these. There are also unused outputs 3 and 4. For some reason, some videos on Facebook and Instagram come out of output 3 in Firefox. When I turn off my audio interface and use the "Built-In" Speakers on my Mac Pro, I hear no sound at all, which leads me to suspect it's erroneously being sent to "Output 3" even when I don't have the audio interface involved at all, making it seem like a Firefox issue. Most videos on Firefox are fine, but it's just some of the videos on Facebook and Instagram. The same videos come out of outputs 1 and 2 in Safari just fine. I tried all the usual stuff- Safe Mode, Disabling Extensions and Plugins, Disabling Add-Ons, Disabling Flash, Disabling the "Block malicious" setting in Flash, Disabling hardware acceleration, Refreshing Firefox, and ''nothing'' fixes this. Why is Firefox doing this?