Some documentation states to enter m.facebook.com and when the popup box comes up to allow notifications, click Allow. I have done that, and it still doesn’t work. Here is how I fixed it.
1. Open Facebook and find and open the video you want to download.
2. In the Address bar change the URL from "www" to "m" and hit enter.
3. Right mouse click the video and choose the option in the context menu that allows you to save the video to your computer.