[vlod]

Mine don't play in FF. I think you can disable this via: media.autoplay.enabled

type about:config in url and set value to false

[frandroid]

I have this set to false and it's pretty annoying. It seems that the same triggers that make auto-play work are used to play regular video. So I always have to fiddle with Youtube to get a video to start, and Vimeo videos don't play at all. I've had to keep from using this flag in Chrome so I have a backup browser for such sites.

[sgruetter]

Thanks, that worked, although YouTube affected, too. Well, better than unexpected auto-play.

[sgruetter]

Update: Does also not-auto-play gifs/gifv on imgur, thus it's not really a solution.