A few months ago, Facebook introduced that 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 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.