Sadly I haven't yet, the same think happens to me, random news videos autoplaying, even when not on screen yet. I'm fairly confident there's no setting to turn off video autoplay on kbin. Sadly even if you tell Firefox to not autoplay videos the videos continue to play, the Firefox mobile settings seem to be limited in that respect though.
I wonder if turning it of on the desktop browser site settings and then syncing will fix it.
Hrmm I think I have automedia preview set to on. I've turned it off and will see if this fixes it. I say think as the UI for the kbin page settings isn't super clear what's on and what's off ..