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. 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:

http://intranet.company.com,http://email.company.lan

Notice that you can use a comma separated list in this field.