The going theory that libcubeb is responsible has been blown out of the water as not the case.
Will have to do GRE-style re-setup in order to do regression build testing. My current hunch is either WebRTC or EME removal.
Regression testing has confirmed EME removal as the point where YT Streams stopped working at commit a3bcbd51d0 even though it literally makes no sense because all that code was ifdef'd under MOZ_EME.
Obviously restoring EME and subsequent GMP should "fix" it but it remains baffling.
Perhaps it should have not been done as one large fuckin commit and done in smaller batches.. But then again, Moonzilla kept ignoring my order to do as such. Given it was done around the time of my move across the country I was unable to review it and trusted it wasn't botched (at the time).
Guess I will have to back it out and re-remove it in a much more responsible manner and properly test along the way.
This if NOTHING ELSE is why I am glad I am no longer a slave to MCP Bullshit. Though I am fuckin sick of these god damned landmines.
I am gonna revert the three GMP removal parts and revert the commit that removed EME.
A follow up would be to ifdef GMP its self as well and find other bits that may be related and test during the process. For now this will align affected files with the old and insecure UXP code though. So it isn't all bad if not merely sub-optimal.
Good news is that WebRTC can stay dead.