Comment by jbaiter

What worked for me was disabling HTTP3 support with the 'network.http.http3.enabled' key in about:config and then restarting Firefox. Seems like it's stuck in the 'SocketThread', repeatedly doing this:

  2022-01-13 08:20:53.075936 UTC - [Parent 4106991: Socket Thread]: V/nsHttp Http3Stream::OnReadSegment count=333 state=4 [this=7f6e295623a0]