Getting HTTP/3 up and running in Firefox Nightly

HTTP/3 is the first major revision of the protocol to abandon TCP, instead running atop the QUIC transport protocol for better performance and security. Firefox Nightly includes experimental support, and it can be paired with Cloudflare's edge network — which has had QUIC and HTTP/3 support enabled since 2019 — for an easy interoperability test bed.

Before you start, note that this is a work-in-progress implementation. Expect occasional rough edges while experimenting. To enable HTTP/3 on your own domain, switch it on in the Cloudflare dashboard under the "Network" tab. From there it's a client-side task.

Enabling HTTP/3 support

Grab the latest Firefox Nightly build from Mozilla's desktop channel page, then open about:config and set network.http.http3.enabled to true. The default values for other parameters are generally fine, though you can filter the config list with a search for "http3" if you want to inspect or adjust anything.

Verifying that HTTP/3 was negotiated

With the preference flipped, rel=oad your site and open Developer Tools (Ctrl+Shift+I on Windows and Linux, Command+Option+I on macOS). The Network tab shows a "Protocol" column identifying which HTTP version served each resource — but only if you enable that column first. Right-click any column header and tick "Protocol."

One caveat: when HTTP/3 gets enabled on a Cloudflare zone, responses carry an alt-svc header advertising the new protocol, but the browser acts on that advertisement only on the next request to the origin. Reload with browser caching disabled (use the "Disable Cache" checkbox or Shift-F5) or you'll see the protocol used during the initial fetch, before the advertisement was received.

If the protocol column still doesn't show HTTP/3:

  • Close and reopen the browser entirely — clients can stick to an existing HTTP/1.1 or HTTP/2 connection, and that's hard to detect manually.
  • Check whether a Service Worker is intercepting requests. Resources served from a local Service Worker cache can look like they came from the network over HTTP/1.1; deregister the worker if you want a true wire-level test.
  • Capture a packet trace and dissect it with Wireshark to independently confirm what's on the network.

For a second opinion, Firefox's about:networking page lists every visited zone with the HTTP version used to load it — a convenient cross-check against the Developer Tools view.

Where the protocol stands

The QUIC Working Group has announced a "Working Group Last Call," a milestone ahead of full standardization. The group reports that all 845 design issues raised against the QUIC drafts now have consensus or a proposed resolution, and that the protocol has become more secure, more widely implemented, and demonstrably interoperable over its development.

Expect implementations to mature alongside the specification, with support eventually moving into stable browser channels. Cloudflare makes guides available for testing HTTP/3 in other clients such as Chrome Canary and curl, plus performance comparisons between HTTP/3 and HTTP/2 and work on congestion control improvements like CUBIC and HyStart++ support in its QUIC implementation.