Try:

• iMacros: https://addons.mozilla.org/firefox/addon/imacros-for-firefox/

Another possibility you may want to give a try is KeePass along with the Firefox Add-on KeeFox. The passwords are stored encrypted, it is open source, integrates nicely in Firefox, has a built-in password generator and you can access your passwords without opening Firefox.

Note that Firefox should be able to store passwords in the Password Manager and fill them automatically.

• https://support.mozilla.org/kb/password-manager-remember-delete-change-passwords

You may have to double-click the user name field in some cases or type the first letter(s) of the name.

cor-el said

Note that Firefox should be able to store passwords in the Password Manager and fill them automatically. [...]

In terms of setting it up this should be the easiest way to go. Additionally, one can easily sync passwords across different devices / firefox instances. And regarding security: It is possible to define a master password which is later the only required password when logging in.