Recent Firefox stopped working

Just last night Firefox stopped working. Specifically, once Firefox attempted to use HTTP/3, the Socket Thread goes nearly 100% CPU usage and no communications happens to the Internet. I saw this with Nightly and Release, and there are reports it happened also to Beta and ESR versions, too.

The Work-Around:

Go into about:config and set network.http.http3.enabled to False.

Then stop Firefox. (You may have to use Windows Task Manager or your operating system equivalent to end the task if Firefox won’t shut down gracefully.)

Then you may start up Firefox and it seems to be table.

I expect the Mozilla developers to come out with a code fix soon, hopefully in the next few days.

Source of fix: Reddit thread PSA: Solution for Firefox not working right now…

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Firefox has released a point upgrade, Release 96.0.1, which fixes one of the two bugs that caused Firefox users grief. It is available through the internal updater.

Once upgraded to 96.0.1, if you had set network.http.http3.enabled to False, it is recommended that you go back and set it back to True.

For those who like technical details, the problem was that Firefox was doing a case-sensitive search for “Content-Length:” in the HTTP3 header but some Mozilla servers (telemetry and upgrade) had recently coded “content-length:” and the difference in case kept it from matching, then an additional bug caused the protocol stack to go into an infinite loop. Release 96.0.1 changed the comparison to case-insensitive. The problem when the content length is missing will have to be handled in a later fix since it is more complicated.