Getting HTTP/3 on Cloudflare with Firefox

After wondering for a good while about a Firefox-specific weirdness I was seeing on Cloudflare-hosted sites, I finally found that there are two solutions. Only thing is, I am able to use only one of those two. That said, I’ll describe the problem and how I found the answers — or, in my particular case, answer, singular.

Note: I originally planned to have more content in this post, but a death occurred in my family during the editing process so I decided to keep this briefer. I hope what does follow will be sufficient for your understanding.

The difficulty I encountered was that, on Firefox¹, Cloudflare-hosted sites wouldn’t show up in HTTP/3, but instead fell back to HTTP/2. (You can test for yourself with the Cloudflare test page for this type of connectivity.) I saw this on both macOS and Fedora Linux. While the performance penalty was likely tiny or nonexistent, it still bugged me and I wanted to find out what was causing this.

I filed a bug in Bugzilla but, if you do so without a Bugzilla account, that automatically generates an issue on the webcompat repo. Mine ended up here and, subsequently, resulted in this Bugzilla issue.

In time, I learned this is related to something called the “Happy Eyeballs” algorithm, Firefox’s handling of which has its own Bugzilla issue. It turned out that there are two remedies for the specific problem I’d seen. One is to use IPv6, and that’s a non-starter for me because my ISP doesn’t support IPv6. The other is to de-activate DNS over HTTPS (DoH) in Firefox; once I did so, that freed up Firefox to “see” HTTP/3 on Cloudflare-hosted sites.

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2025-10-30 at 12:53:42 PM CDT.
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