Just a heads up, we tried to enable H2 with haproxy 1.8.3 last week and had to roll it back because of a Firefox bug. I believe it's this one, which doesn't seem to be getting much attention: https://bugzilla.mozilla.org/show_bug.cgi?id=1427256
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.
Glad to see the bug is now updated/fixed, but the person you're replying to said they were already using 1.8.3. I guess you're saying their issues were caused by a different bug.
That bug is in HAProxy. The RFC clearly states that the presence of the Connection header makes the response malformed.
"An endpoint MUST NOT generate an HTTP/2 message containing connection-specific header fields; any message containing connection-specific header fields MUST be treated as malformed (Section 8.1.2.6)." and "Clients MUST NOT accept a malformed response. Note that these requirements are intended to protect against several types of common attacks against HTTP [...]"
(though Safari's response of repeatedly retrying as fast as possible is certainly problematic)
