If you clicked "Never" then you have created an exception that you need to remove.

Remove site(s) from the Exceptions:

• Tools > Options > Security: Passwords: Exceptions

• http://kb.mozillazine.org/Password_Manager
• https://support.mozilla.com/kb/make-firefox-remember-usernames-and-passwords

I didn't click on any of the 3 choices. They just disappeared before I was able to make a choice. I looked and there are no exceptions in the security area.

The website may be using autocomplete=off to prevent Firefox from saving the name and password.

You can remove autocomplete=off with a bookmarklet to make Firefox save the name and password.

• http://kb.mozillazine.org/User_name_and_password_not_remembered

• Saved Password Editor: https://addons.mozilla.org/firefox/addon/saved-password-editor/