Update: Firefox says the problem has now been resolved: “Earlier today, Firefox became unresponsive due to a change in defaults by a cloud provider which triggered a Firefox HTTP/3 bug. We disabled the configuration change and confirmed this fixed the issue. If you’re still affected, please restart your browser. We’re sorry for any inconvenience caused.“

A bizarre bug in Firefox has cropped up that means users of Firefox are unable to load any website in the Mac, Windows and desktop Linux versions of the browser. While the developers work on a solution to the bug, there is a fix you can apply to workaround the issue …

Affected users are simply seeing their tabs spin forever, never completing loading to show the page.

The underlying issue preventing pages from loading appears to be an infinite loop bug in Firefox’s HTTP3 implementation. This essentially causes the browser process to hang indefinitely.

In particular, the current theory is that one of Firefox’s data analytics services recently upgraded to a HTTP3-powered backend and the browser is failing to resolve it successfully. The actual implementation bug will have existed for a while but it has only come to the fore because Firefox attempts to connect to data analytics servers almost immediately after launching the app.

Luckily, very few real websites actually care about HTTP3 with almost everyone either still using older standards, or providing a fallback. This means HTTP3 capability is not yet required to browse the modern web. The workaround to the bug then is to simply disable HTTP3 loading entirely in Firefox. To do this:

  • Open a new Firefox window.
  • Type about:config in the URL bar. This will open a settings screen.
  • Search for the setting ‘network.http.http3.enabled’.
  • Set this setting to ‘false’ to disable HTTP3.
  • Then, fully close and restart Firefox.

This workaround applies to all desktop versions of Firefox, so it’ll work regardless of whether you are on Windows and Mac.

Of course, once the actual software bug has been resolved, you’ll want to go back and re-enable HTTP3 so that you can use HTTP3-dependent services in the future.

(The iOS version of Firefox is not affected by the bug as its underlying rendering engine is WebKit, the same as Safari. This is because on iOS, Apple does not allow third-party browsers to use their own engines for security reasons).