First of all go to Chrome webstore (click here) and install this Extension ( there is "+Add to Chrome" button on extension page click that button to install ). After installation move to step one ( note: make sure to refresh facebook if facebook is already opened in another tab if facebook is not already open then just open it no need of refreshing )
Open facebook and just go to any video you will see download button near the video move your cursor to download button
once you move your cursor to download button you will see download option ( in SD and HD ) in dropdown click your desired quality button
If you can't see Download button near video then simply just click our extension icon at the top right (check the screenshot below)
After you click the extension icon you will see all the videos with download button in popups just below the extension icon ( See the screenshot below ).