Disable the password manager and use save form data to store the user name.

• Tools > Options > Security: Passwords: [ ] "Remember passwords for sites"
• Tools > Options > Privacy > History: [X] "Remember search and form history"

To see all the History and Cookie settings, select:

• Tools > Options > Privacy > History: Firefox will: "Use custom settings for history"

What about setting for "Remember Browsing History"? Do I need this to be anything other that 0 days?

In Firefox 3.5 and later 'Form and Search history' entries expire just like the regular 'Browser History'.
The pref browser.history_expire_days determines when history items expire and also effects form and search data.
See http://kb.mozillazine.org/browser.history_expire_days (180) (also affects saved form data)

Make sure that you do not use Delete browsing, search and download history on Firefox to clear "Form & Search History"

Thank you. That was the missing part.

Just for people who step about this topic and have all checkmarks enabled:

FF does not save your username/password IF the form is not submitted by a submit button, i.e. like this:

<form..><input ...> <a href... onclick='login()' class=loginbutton>Loginbutton</a> </form> .

In this case, firefox does not offer the save password option, nor does it save the entries made. That's because it's not a form submit at all, but it looks like one to the user.

@Webdeveloper:

In that case exchange the "<A>..</A>" with something like this: "<input type=submit class=loginbutton onClick='return login();' name=submit value='login'>"