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)

Does not work. On any YouTube video, clicking the download button just results in "Cannot detect url_encoded_fmt_stream_map or adaptive_fmts"

useless, does not work!

The best!!

Works well enough and integrates pretty nicely into youtube.

Sometimes the extension will stop working because youtube changed a few things. When that happens, updates usually seem to arrive within a few days.

I've seen some people complain about this extension not offering video+audio streams in certain resolutions on certain videos, but that's hardly developer's fault (since they just "expose" the download links that youtube player uses. If a stream of certain resolution and/or audio quality is missing, it's because Youtube no longer supports that format of stream).