I am trying to access the test page https://cloudflare-quic.com which is QUIC and HTTP/3 enabled and my Firefox Nightly version is 89.0a1 (2021-03-24) 64bit.
My Firefox Nightly configurations relevant to QUIC are as follows:
network.dns.httpssvc.http3_fast_fallback_timeout=10000
network.http.http3.alt-svc-mapping-for-testing [unset]
network.http.http3.backup_timer_delay = 100
network.http.http3.default-max-stream-blocked = 20
network.http.http3.default-qpack-table-size = 65536
network.http.http3.enable_0rtt = false
network.http.http3.enable_qlog = true
network.http.http3.enabled = true
When I visited the test page at https://cloudflare-quic.com the first time the page shows that my browser is using HTTP/3 which is great. But when I refreshed the page using Ctrl+Shift+R, it suddenly downgraded to HTTP/2. I have already modified the fast fallback from the default 50 to 10000, and disabled 0rtt. I have no service workers registered or have any page caches.
My question is: Why is my browser still using HTTP/2 although that page already advertised alt-svc: "h3-29":":443" ?
