A few months ago, Facebook introduced that “cool” new feature where videos your friends posted would automatically start playing as soon as you scrolled to them on your newsfeed. I don’t know why they thought this was a good idea or if it was part of their philosophy that people are incapable of choosing what they want to see without some kind of automation involved but they must have finally gotten enough complaints about it because there’s now a new feature that will stop it.
In your Settings panel (click the ∇ in the very top-right part of your FB page and choose “Settings”), click on Videos and then set the “Auto-Play Videos” setting to “Off”. That’s it! Now scroll through your news feed and verify that that videos are no longer automatically starting.