It had the exact same problem almost 1 year ago. The option activated silently without any warning on the stable channel, even with mozilla's "experiments" completely disabled. It was kinda hard finding out what the culprit was, since it was intermittent.
"Recent Firefox Nightly with HTTP3 enabled has problems loading HTTPS sites on #Cloudflare"
Thanks to this bug report by @cks I was able to find a solution.
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