In the Firefox browser:

• Click on Video.
• Click on Saved Video.
• Click the three-dot ellipsis menu on a video and select Copy link.
• Paste that link into a new browser tab. and watch it forward the shortened link (starting with https://fb.watch/) to something that starts with https://www.facebook.com/watch/.
• In the address bar, change the “www” to “mbasic.”
  • That forces the browser to load the mobile version of the page for you.
• Right-click the video, and select Open link in new tab.
• Click the Firefox menu, and select “Save Page”.
  • This should show the save dialog box with the video as blahblah.mp4.
  • Rename as needed.