For the time being, there are a few workarounds you can try:
1. Use another browser.
3. Temporarily disable HTTP3 (about:config > network.http.http3.enabled > false)
For those who choose option 2 or 3:
Please keep track of this issue and re-enable the settings once this outage is completely fixed.
Thanks for all the Firefox users who make the effort to file a bug report. There are nearly 30 duplicated reports already. Please refrain from filing another one.
UPDATE:
Root cause may be HTTP3. Mozilla has disabled HTTP3 for their telemetry (data collection) servers, i.e. option 2 is no longer relevant; you may re-enable it.
If issue persists, please use option 3.
UPDATE2, from the BMO bug report:
Firefox has witnessed outages and we are sorry for that. We believe it's fixed and a restart of Firefox should restore normal behaviour. We will provide more information shortly