So you've found a video on Facebook that you either want to keep (download to your computer), post on another website, and/or convert to some other format (for free).
There's an easy way to do any of these. You'll be needing:
Once you've installed the Firefox extension, and restarted your browser, go to the video on Facebook that you want. You'll see these new links added by the extension:
Clicking on "Convert Video" will take you to another website to convert the video, with all of the video information pre-filled for you.