FaceBook have recently made changes so that some videos auto-play. This is not a problem if you have a cable internet connection. However if you access the internet using a mobile dongle which charges you per MB then you will be wasting your bandwidth downloading videos which you may not be interested in watching.
To disble autoplay the first step is to log into your FaceBook account and then click on the little triangle symbol in the top right hand corner of your screen then select SETTINGS from the little menu which appears:
You then need to select VIDEOS from the menu down the left hand side of your screen:
This will take you to the Videos settings page where you need to change the settings from ON to OFF:
This should now stop videos auto-playing and so save you downloading MB you may have no interest in watching.