Short answer: go to this link https://www.facebook.com/settings?tab=videos and click on ON/OFF. Done!
Isn’t it annoying that facebook video autoplay everytime you scroll or mouse over? well for the many of us, it is annoying. Sometime we don’t want to see that video auto play and sometime we do. The impact with autoplay also will cause your computer to slow down, error out, due to the computer usage, yeah some of us still have the old computer which is enough for daily usage internet email videos and so on.
For me personally, I don’t really like facebook auto play. Mainly because I’m subscribed to a couple or more fan page that have videos and I want to be in control, be able to play when I want, not all the time with the mouse over. I don’t know why facebook made it as a default to play when you mouse over or scroll to that particular video. Most of the time this caused my browser, Chrome, Firefox, IE, freeze, and that’s not a good thing. Bad functionality facebook! disable it. The majority of the users does not like this option.