Creating a Firefox extension to edit "network.automatic-ntlm-auth.trusted-uris"

On the sharepoint site we develop at work, people can log into the site using IE 7, but when prompted for a username and password on firefox 3, it doesn't matter how many times you try to login on, the username / password is revealed. If you click the "Cancel" button, it denies you access to the site. We tried to fix the problem for our individual machines:

1) log into IE 7 by clicking "save password" when logging in.

2) Then go to firefox by typing about: config,

3) URI lookup

4) Adding ".csom.umn.edu" to the value column for the three results that appear.

This is a long and complicated procedure for our clients, is it possible to create an extension or add-on that automates this procedure by simply loading it after the first login failure with firefox?

Thank!