I'm giving this a priority so we can do a bug breakdown. Byron -- would it be reasonable (time-wise) for you to do a bug breakdown on this between now and the end of this month (November)? Or are you too swamped?
One issue that came up recently when abr was helping a developer: The dev made changes to the SDP that we chose not to act on, but we currently fail silently instead of making setLocalDescription fail. That would be a good first bug to file (blocking this bug) and fix.
We have a partial breakdown in the code over here: https://dxr.mozilla.org/mozilla-central/source/media/webrtc/signaling/src/jsep/JsepSessionImpl.cpp?case=true&from=JsepSessionImpl.cpp#1773 Some time/thought needs to be put into this. I suspect simulcast is going to keep me pretty busy.
Given our other priorities, I need to lower this slightly (to a high P2).
Mass change P2->P3 to align with new Mozilla triage process.