If you'd like to follow developments on this the Firefox bug seems to be this one : https://bugzilla.mozilla.org/show_bug.cgi?id=1749908
Someone from Firefox posted :
Our current suspicion is that Google Cloud Load Balancer (or a similar CloudFlare service) that fronts one of our own servers got an update that triggers an existing HTTP3 bug. Telemetry was first implicated because it's one of the first services a normal Firefox configuration will connect first, but presumably the bug will trigger with any other connection to such a server.
This means that disabling Firefox telemetry might not be enough to resolve the issue if you access other HTTP/3-enabled services.
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