Facebook has an option to share videos with friends. If you want to download those videos to your system, there are many tricks and downloader tools are available.
This post will tell you about an Add-on available for firefox.
Follow these steps to download videos from facebook with firefox
download add-on https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/
install it in firefox and restart browser after installation
login to your facebook account and play the video you want to download.
Click on the Red U arrow mark on add-on to download the video.