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If you set 'network.http.http3.enable' to 'false' in 'about:config' the site loads and runs smooth as butter again. Firefox has had HTTP3 implementation since 2020 (enabled by default since 2021) so I'm not sure why it's acting up now. Maybe something was forced or something deprecated in Firefox.

EDIT; I just noticed another reply further up also mentioning this. That's what I get for skimming. Sorry for adding nothing to the conversation. lol