Works OK, dont know why the dev doesnt give proper instructions. There is a button UNDERNEATH timeline FB posts (but not in 'saved'). Click this button and you are taken to their web site, click download and FF download dialog opens.
Toolbar button opens their web page, you paste the URL, click "HD", it opens a new tab which plays the vid. No downloaded file. Would help if they made it clear you can SAVE THE NEW TAB (it's the extracted mp4 file).
Quality seems about the same quality you see playing it on FB, no info about the quality of vids available