Computer Configuration > Administrative Templates > Firefox > Authentication
Both
HKLM\Software\Policies\Mozilla\Firefox\Authentication\AllowNonFQDN
If this policy is enabled (and the checkboxes are checked), you can always allow SPNEGO or NTLM on non FQDNs (fully qualified domain names). If this policy is disabled or not configured, NTLM and SPNEGO are not enabled on non FQDNs.