If you fail to download videos from Facebook, please refer to the following solution to fix it.
1. Make sure you are using the latest version. Click on Preferences > Check for New Version to check it for Windows. For Mac, click on program name on the top left corner of your Mac and click on Check for Updates...

2. If you still fail to while using the latest version, please go to Preferences > Extensions to install the Download button for your browser and then refer to the Download button appearing above the Facebook video to download it.

If you still fail to, please provide me the video URL in problem and click the Feedback button on the top right corner of the program to provide us the info and we'll back to you soon with the solution.