EDIT 1-25-2020 11:25AM Eastern
Just one day later and this is working again. YAY!! feller ROCKS

signatureLocal: cannot resolve signature;3

Was awesome, now it does not work at all. I'm sure it will be updated so keeping the 5 star rating, for now.
Not a new problem, YT changes stuff all the time. This developer keeps changing his app to comply with the changes.

No proper funktion since the last days ... HOPE for update soon!?

This was a 5 star add-on until this latest update. Now I get this error:
signaturelocal: cannot resolve signature;3

All of a sudden, it flat out doesn't work. I guess either a bug was introduced or Google eliminated direct downloads period, not just in their own store.

Have uninstalled and reinstalled the extension, cleared cache, refreshed Firefox, etc Still no joy.

Would rather not be forced to pay for a separate downloader to get the functionality I had as little as a day ago.

UPDATE: 05/07/2020
Now the new error is "Type error for parameter options (Error processing url: Expected string instead of undefined) for downloads.download.

Went backwards to FF 75 from 76 and the same error.

Thank you so much. I was finally able to download a youtube video. I tried other down loaders but they didn't work.

This version (0.9) stops working, if I move its icon from the toolbar to the overflow menu. Tested with Firefox 72.0.1 x64 with different profiles on Windows and Linux.
Except this, it is working perfectly.

everything works just fine, best YT downloader I've tried so far

Sorry, but not very useful as it only presents a limited range of resolutions that are video & audio and theyre all either too low (360p) or too igh (720p+). Im used to being able to get 480p (perfect for me on limited data and mobile) which i usualyl get via IDM...currently im in Linux and if course no easy IDM downloads, so trying all Add Ons for youtube. And so far, all are so limited....this one is just weird, theres 1 page with limited video&ausio options and 2 pages of audio or video only options...bonkers....

Excellent extension. Does what was promised. Probably the best. YT or FF makes changes, but the Feller makes it work again and constantly improves it.

It demonstrated high efficience, however it is now requesting change of format. I wonder why.

so so so excellent!!