Bugzilla Bug #1749908 has the gory details.

Workaround

Disable HTTP3 support by entering in the Firefox Address / Search bar:
about:config
Enter into the Search box:
network.http.http3.enabled
Change the value to 'false'
Restart the browser (force restart, or reboot device even, if necessary)
Chrome users not affected
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And this is what happens when you go against Bitcoin :D
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I'm a former firefox user but the web is now converging on the Blink browser engine. Besides ideological considerations are there spots where firefox has certain usecases vs chromium?
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Eventually, this needs to to be re-enabled (i.e., value toggled back to 'true'), but until Firefox fixes their issue, doing that prematurely would likely cause you to hit the freeze issue once again.
The risk to leaving it disabled (i..e, value as 'false') permanently is that eventually at some point Firefox will depend on http3 being enabled.
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