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Do you have some sort of playlist/media downloading add-on enabled on Firefox? Because that is what the extension listed is for.
I'm using Firefox as well and don't get that, the next thing I would look at is a possible malware infection.
Edit: changed extension to add-on. I've got chrome on the brain at the moment.
.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.