Hmm…do you have the in-browser DNS over HTTPS resolver enabled? I personally can't reproduce this, but I'm using DoH with 1.1.1.1.
I've noticed that both Chrome and Firefox tend to have less consistent HTTP/3 usage when using system DNS instead of the DoH resolver because a lot of times the browser is unable to fetch HTTPS DNS records consistently (or at all) via the system resolver.
Since HTTP/3 support on the server has to be advertised by either an HTTPS DNS record or a cached Alt-Svc header from a previous successful HTTP/2 or HTTP/1.1 connection, and the browsers tend to prefer recycling already open connections rather than opening new ones (even if they would be "upgraded" in that case), it's often much trickier to get HTTP/3 to be used in that case. (Alt-Svc headers also sometimes don't cache consistently, especially in Firefox in my experience.)
Also, to make matters even worse, the browsers, especially Chrome, seem to automatically disable HTTP/3 support if connections fail often enough. This happened to me when I was using my university's Wi-Fi a lot, which seems to block a large (but inconsistent) amount of UDP traffic. If Chrome enters this state, it stops using HTTP/3 entirely, and provides no reasoning in the developer tools as to why (normally, if you enable the "Protocol" column in the developer tools Network tab, you can hover over the listed protocol to get a tooltip explaining how Chrome determined the selected protocol was the best option available; this tooltip doesn't appear in this "force disabled" state). Annoyingly, Chrome also doesn't (or can't) isolate this state to just one network, and instead I suddenly stopped being able to use HTTP/3 at home, either. The only actual solution/override to this is to go into about:flags (yes, I know it's chrome://flags now, I don't care) and make sure that the option for QUIC support is manually enabled. Even if it's already indicated as "enabled by default", this doesn't actually reflect the browser's true state. Firefox also similarly gives up on HTTP/3, but its mechanism seems to be much less "sticky" than Chrome's, and I haven't had any consistent issues with it.
To debug further: I'd first try checking to see if EncryptedClientHello is working for you or not; you can check https://tls-ech.dev to test that. ECH requires HTTPS DNS record support, so if that shows as working, you can ensure that your configuration is able to parse HTTPS records (that site also only uses the HTTPS record for the ECH key and uses HTTP/1.1 for the actual site, so it's fairly isolated from other problems). Next, you can try Fastly's HTTP/3 checker at https://http3.is which has the benefit of only using Alt-Svc headers to negotiate; this means that the first load will always use HTTP/2, but you should be able to refresh the page and get a successful HTTP/3 connection. Cloudflare's test page at https://cloudflare-quic.com uses both HTTPS DNS records and an Alt-Svc header, so if you are able to get an HTTP/3 connection to it first try, then you know that you're parsing HTTPS records properly.
Let me know how those tests perform for you; it's possible there is an issue in Firefox but it isn't occurring consistently for everyone due to one of the many issues I just listed.
(If anyone from Cloudflare happens to be reading this, you should know that you have some kind of misconfiguration blocking https://cloudflare-quic.com/favicon.ico and there's also a slight page load delay on that page because you're pulling one of the images out of the Wayback Machine via
https://web.archive.org/web/20230424015350im_/https://www.cl... when you should use an "id_" link for images instead so the Internet Archive servers don't have to try and rewrite anything, which is the cause of most of the delays you typically see from the Wayback Machine. (I actually used that feature along with Cloudflare Workers to temporarily resurrect an entire site during a failed server move a couple of years back, it worked splendidly as soon as I learned about the id_ trick.) Alternatively, you could also just switch that asset back to https://www.cloudflare.com/img/nav/globe-lang-select-dark.sv... since it's still live on your main site anyway, so there's no need to pull it from the Wayback Machine.)
I've spent a lot of time experimenting with HTTP/3 and it's weird quirks over the past couple of years. It's a great protocol, it just has a lot of bizarre and weirdly specific implementation and deployment issues.
