Firefox needs to save the old passwords so a user can lookup previously used passwords. Now that many work/medical/financial websites require routine updates to passwords, I need to be able to see 3 or 4 versions back so I don't accidentally reuse one too soon. I was trying to look up an old password and it would be really convenient if I could see previously used passwords once I was logged into my browser. Maybe you protect that view with a application level password so a visiting user can't peek at them.