Now this is really weird! I just clicked on the link my original post to the Samantha Mohr video to make sure it was correct and, much to my surprise, I saw a brief "re-directing to ..." message before the video loaded and was detected by DLH. I then tried the Weather Nation link and the same thing happened - message about re-directing followed by the video loading, playing and being detected by DLH.
So I experiment further, and I think I have found a work-around for the problem:
1) Play the FB video in your news feed (or wherever else it is on FB)
2) Pause the playback and right-click, and from the pop-up menu click show video URL
3) Click in the small window that opens showing the URL, right-click and then click "copy"
4) In a new Firefox tab, Paste & Go the copied URL
5) You may or may not get the "re-directing..." message, but the video should load and play in the new tab.
6) Within a few seconds DLH should detect the video, allowing you to select the version (if any - resolution) and download as usual