If you click on the username field, and use the up and down arrow keys, does it appear in the drop-down list? If so, you can delete the old one by selecting it and press the Del key.

By the way, you should update to the latest version of Firefox. See Update Firefox to the latest release

If you delete the name then is is possible that you still have an entry left with an empty name. You can check that in the Password Manager. Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords"

Thanks guys but I am still stuck.

I have tried clicking in the username field and then using the up and down arrow keys but still no drop down menu.

I have downloaded the latest version of firefox 3.6

I have checked in the password manager and there is nothing there.

Surely there must be a way of requesting that your username and password will be remembered?

Are you in private browsing mode? See Usernames and passwords are not saved

Thanks Chris - It's working now :)