First, find the video on FB that you wish to save. Click on it and on the right hand corner of the page, look for the've seen the share button.
Choose Copy Link from the Share options. When that is done, into a facebook video downloader of your choice, the link you just copied should be pasted wherein after that, you would have to click a button that says download in order to download the video in high quality.
Do it now and watch your favorite FB videos offline whenever you feel like it.