The extension no longer accepts passwords. Most likely it has to do with new firefox update. You enter correct password and instead going to the page you intend(i.e. addons) you get password page again.
I'll also raise an issue on Github

Sadly, today july 29, 2020 firefox updated the browser and this add on is broken now miserable, it won't accept the password you set on new version of firefox :(

the only downside is its preventing me to play my facebook videos...