.m3u8 is a file format for video streams. It works like a playlist, it's a file that defines a collection of individual video files, each being one small part of the stream.
m3u8's can't be played natively by a browser, and if the OP has enabled autoplay media, kbin is probably directing the browser to that URL, the browser can't play it in-tab, so it downloads it as a file instead.
Quirky little bug you found there OP. Codeberg would love to know.
Actually, it just did it again as I was scrolling through All...anytime this post about the debris field near the titanic loads, it downloads the file. I'll search through my firefox settings and see what I can see.
I don't believe so. The only extensions I have are bitwarden, multi-containers, uBlock, and Dark reader. It actually has stopped now after I logged out of Kbin and then back in. I suppose I'll associate it with some sort of user error, because I don't see the problem being posted aside from me. Thanks for the suggestions