Isso usa a autenticação NTLM?
Se sim, então você precisa ativar o repasse de NTLM no Firefox:
link
- Open Firefox and type
about:configin 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.lanNotice that you can use a comma separated list in this field.