Password only filled after entering user name
From MozillaZine Knowledge Base
This article describes how to make Firefox fill in user names and passwords when a page loads rather than only filling in a password after a user name has been entered. If you're having trouble getting user name and password remembering to work at all, see User name and password not remembered.
Multiple user names
If you have multiple user names saved for a site, then the user name will not be filled in automatically but you can double-click the user name for a drop-down list (all versions), and select the user name (user-id) wanted, and the password will also be filled in if remembered.
For some sites such as Google and Yahoo the password might not get filled in even if remembered in which case try "sign in as a different user" even though you re-enter as the same user, after entering your master password for entry and supplying your user name, the password for the site should then be filled in if remembered.
If you still have a problem try "Tools -> Options -> Security", click "Saved Passwords..." (Firefox 3 and later) or "Show Passwords..." (Firefox 2), and ensure that only one user name is defined for the problem site.
Prefill disabling preference
Firefox 2 versions above 126.96.36.199 and Firefox 3 and later versions have a preference, that allows you to disable pre-filling user names and passwords. By default, this preference is set to true, which allows pre-filling. Make sure this preference is set to true if you want Firefox to pre-fill the fields. You can set the preference to false if you want to disable this feature.
Firefox 3 and later: