Firefox: one form multiple password fields

As far as I know, there is no way to do what you want with the default Firefox setup. The pop-up authentication will only remember the most recent password, not giving you a drop-down like it does with in-page remembered passwords.

IIRC, you can save it for that one site then disable the Password remembering. Another way, since Firefox's password manager is so insecure that you can access it without even booting into your OS, even with a master password, is to use KeePass, which fakes keystrokes to enter your password in on-demand. Much more secure.