No idea, if I see a medium link I just ignore it. Substack is heading the same way for me too, it seems to be self-promotion, shallow-takes, and spam more than anything real.
The page loads a "subscribe to author" modal pretty quickly after the page loads. You may have partially blocked it, so you won't see the modal but it still prevents scroll.
Firefox has a lot of weird little pop up ads these days. It seems like this is a very recent phenominon. Is this actually Firefox doing this or some kind of plug-in accidentally installed?
Same. Hit escape shortly after the page loads to stop loading whatever modal is likely blocking scroll. I don't see the modal so it's likely blocked by ublock, but still stops scroll.