On that bug, it looks like the reporter has yet to give an update detailing what the bug actually is (the connection header is a mislead). Their update on Dec 30th says "I'm still working closely together with Willy from Haproxy to determine the actual root cause of http2 failures in Haproxy which only seems to affect Firefox". Without information on the actual fault, it seems like there's little the Firefox developers can do. It could easily be a Haproxy bug.
