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, Firefox won't know which to use. Check "Tools -> Options -> Security -> Show Passwords" and ensure that only one user name is defined for the site you're having problems with.

Prefill disabling preference

Firefox 2.0.0.1 and above have a preference, signon.prefillForms, 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.

1. Type about:config in the Location Bar.
2. Press Enter.
3. Find the preference name signon.prefillForms.
4. Double click on it to set it to true.