Nightly tries to download a file named master.m3u8 which is a Playlist for the actual video that plays.
(I’m talking about the embedded video right here in this page, not when following the link.)

And yes uBlock Origin everywhere all the time, otherwise browsing is simply a pain.

Weirdly, Firefox doesn’t autoplay if I reload the page, but if I follow a link here (like from the replies page) it does.