Do you have an Intranet or a similar web site that requires the use of Integrated Windows Authentication? If so the default Firefox browser settings will always prompt you for a username and password first before accessing a site using Integrated Window Authentication.
Fortunately Firefox has the slick ability to easily modify it's configuration to use Integrated Windows Authentication.
Configure Firefox
To enable windows authentication on your domain.
1. Open Firefox
2. Navigate to the url about:config
3. Locate the following preference names and put as the value the comma separated values of the address roots.
network.automatic-ntlm-auth.trusted-uris
network.negotiate-auth.delegation-uris
network.negotiate-auth.trusted-uris
Your value should look something like these localhost, server1, server2, serverX.. etc.