Developer updated their product. Product still doesn't work (for me). Don't get me wrong, the product functions on "normal" videos that aren't aren't altered upon upload. But if the uploader alters the video in any fashion which changes how YouTube indexes the said video, the downloader fails even function. The interface won't even show up on the page.

Example: Say a video goes up and you save it to a favorite playlist. The uploader than decides to change the video from "public" to "unlisted." Because of how it's indexed, it throws a HTTP 410 error. And while the URL itself will still redirect to the video in a browser, the downloader (and every type of of player/recorder I've tried) all throw the same code. So the short answer is you'll have to screen capture videos if you follow creators that make a habit of this. I'm not sure if the developer has a workaround solution for this type of behavior which Mozilla (and browsers in general) account for it at the application level when it parses info through the OSI model that they can build in to their addon. It would definitely be nice.