How to fix Firefox if you're having trouble today :

  1. Kill the running Firefox process (through your favourite task manager, htop, whatever)
  2. Access about:config
  3. Search for network.http.http3.enabled and set it to false
  4. Close Firefox and kill it again
  5. Launch Firefox
  6. Enjoy
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If you'd like to follow developments on this the Firefox bug seems to be this one : bugzilla.mozilla.org/show_bug.

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Some people have reported disabling all Telemetry from Firefox (Firefox Settings > Privacy and Security > Firefox Data Collection) also works around the issue, but the person who originally posted this since then seems to have deleted it's post, take this with a grain of salt.

I'm confirming simply deactivating all Firefox telemetry is enough:

  1. Open Firefox Settings
  2. Go to « Privacy and Security »
  3. In « Firefox Data collection », uncheck all checkboxes.

If you have changed the network.http.http3.enabled setting as told above, you can reset it to it's initial value (true).

Restart Firefox to finish.

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.

Our current plan is to disable HTTP3 to mitigate until we can locate the exact bug in the networking stack.

This means that disabling Firefox telemetry might not be enough to resolve the issue if you access other HTTP/3-enabled services.

@tcit turning telemetry off and HTTP3 back on worked for me

@Siph yeah, I waited a while after testing just to be sure :)

@tcit Not sure this is enougth as per bugzilla.mozilla.org/show_bug. you can also trigger this issue with Google Cloud http/3 load balancer.

@tcit this technically isn't enough until the underlying http3 bug is fixed
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