You need to enable NTLM Configuration within FireFox. It is very simple to do and should solve your problem:
- Open Firefox and type “about:config” in the address bar. (without the quotes of course)
- In the ‘Filter’ field type the following “network.automatic-ntlm-auth.trusted-uris”
- Double click the name of the preference that we just searched for
Enter the URLs of the sites you wish to pass NTLM auth info to in the form of:
Notice that you can use a comma separated list in this field.