Hacker News Reader

That's what I guessed, but the http3 fix has no effect

Changing user-agent doesn't do anything either

Spoofing the user-agent to look like Chrome made reddit work for me when using Firefox.

Weird - perhaps the extension I was using didn't actually work

Seems like they've fixed whatever it was now anyhow

It seems the User-Agent workaround solved it for some people, but not for others. Definitely weird.