Hi   !

Unfortunately the developer of that add-on hasn't updated since November 2016 - you could try to contact him. See the first post in this thread by TylerDMozilla (Mozilla employee) :

https://www.reddit.com/r/firefox/comments/6th6fv/updated_firefox_and_your_addons_say_legacy_heres/

Should that add-on no longer work with the release version of Firefox, you could switch to the ESR version (till mid 2018) :

https://www.mozilla.org/en-US/firefox/organizations/

Note that you can double-click the username and password field in the Password Manager to edit these fields. You only can't ass a new password when the website wouldn't recognize the login field or when you need to create a special form.