Which indicates the credentials are fine as is the proxy part of the transaction. Firefox appears not have security access to the OS properly to do the background stuff required. 2/3 of NTLM and related protocols is done in background actions.
If it's working in IE then its probably one of Firefox's NTLM settings. If you enter "about:config" in the address bar of FF and then filter for "ntlm" you will see what options are available.
More than likely be the "network.automatic-ntlm-auth.trusted-uris;" option needs the address of the app server listed.
Unfortunately I've checked network.automatic-ntlm-auth.trusted-uris and
the website hostname is in there also. Very frustrating!! Firefox
doesn't appear to be able to auth via either method, IE will
successfully auth via Kerberos.
