1. Right click on video while its playing and click show video url.
2. Copy generated video URL.
3. In my case, it is https://www.facebook.com/iamkamalnayan/videos/651573224985442/
4. Replace www to touch in the URL and open in new tab (or existing one).
5. Old URL: https://www.facebook.com/iamkamalnayan/videos/651573224985442/
6. New URL: https://touch.facebook.com/iamkamalnayan/videos/651573224985442/
7. Play the video.
8. Right click on video, while its playing.
9. Click on Save Video As to save the video.