Computer Configuration > Administrative Templates > Firefox > Authentication
Both
HKLM\Software\Policies\Mozilla\Firefox\Authentication\NTLM
If this policy is enabled, the specified websites are trusted to use NTLM authentication. Entries in the list are formatted as mydomain.com or https://myotherdomain.com. If this policy is disabled or not configured, no websites are trusted to use NTLM authentication. For more information, see https://developer.mozilla.org/en-US/docs/Mozilla/Integrated_authentication.