The Firefox Password Manager securely stores the usernames and passwords you use to access websites and then automatically fills them in for you the next time you visit.
When you click on "Never Remember Password for This Site"
for a site, that site is added to an exceptions list so passwords will not be saved for it, and you will no longer be prompted to remember the username and password when you sign in to the website in the future.
When you remove a site from exceptions
, the next time you sign in to the site for the first time again, you will be prompted to remember it's username and password.
This tutorial will show you how to see and remove exceptions
for all or specific sites that you used "Never Remember Password for This Site"
for in Firefox