A friend has uploaded a video to Facebook and you want to download the video for future entertainment or other reasons?
This is absolutely doable and easy!
Videos that are uploaded by other users are hosted by Facebook.
Facebook has no controls to download videos though so you will need a third party plugin.
I can very much recommend the FireFox plugin "Easy Video Downloader" which is excellent for downloading videos on Facebook, YouTube and other websites.
First, open your Firefowx browser, if you don't have it installed on your computer you can download it here for free.
Now in the top menu, go to > Extra and click on Add-ons. Type Easy Video Downloader in the search box and click "install"
After istallation you will find an arrow in your browser's navigation bar.
So how to download a video from facebook?
Go to Facebook and play the video that you want to download. You don't have to play the whole video, just a second is enough.
Click on the arrow in your browser's navigation bar and click on the video file you wish to download.
This will work for all Facebook videos and most other videos you will find on the internet.