Was working perfectly, but youtube has now changed how they deliver videos.
Waiting patiently for the various video downloaders to come up with a workaround.

Update, have just revisited youtube and the extension now seems to be working again, perhaps google have reinstated whatever it was that broke the downloaders earlier.

update 20201104
new version fixes the annoying permissions and other errors that have happened since the FF update from v81 to v82 :)
The best youtube video downloader is back (happy dance)