This works perfectly. I don't have any complaint about it.
Only I'd hope that it will develop soon to somehow let us encrypt the secret keys, maybe creating some kind of user locally (to login once every session as the browser is opened). That would be amazing!

Updated review. June 2019

Mozilla doesn't allow new comments, so I have to edit this same review.
Now this wonderful add-on has encryption and it works good, except for one thing:
For some reason when I type my password on the add-on, it doesn't hide the letters. It should show only asteriks (*) but it doesn't.
It is a problem. I don't want people around me to know what's the password of my authenticator. Now I must look around and type when I believe no one is looking at the screen.
Nontheless, when I enter the preferences page of the extension (in the firefox administration page for add-ons), there it does hide my letters and shows only asteriks...
I've downgraded the rating to three stars, but when this is solved I will upgrade it to five again. This little extension is lovable.