Has worked reliably for years until two weeks ago when I downloaded a now-banned video. I can no longer see the download button. Since then, I have tried to use other YouTube download add-ons, but they do not work either, so maybe the problem is on the Mozilla side or YouTube side. I see that there are "Mozilla recommended" add-ons, but they require invasive levels of permissions.

UPDATE: I uninstalled and reinstalled. Now working. (Disable-/re-enable did NOT work).