You could get to it through about:config (so.. a little difficult for the common user to reach)
media.autoplay.enabled
media.autoplay.allow-muted
They then changed to
media.autoplay.default
media.autoplay.block-webaudio
media.autoplay.enabled.user-gestures-needed
And removed the previous parameters, which caused the videos to start playing automatically again, until you would configure the new properties (again, only though about:config).
Finally, they're now providing a menu option for achieving the same, which deserves to be applauded.
