Computer Configuration > Administrative Templates > Firefox > Authentication
Both
HKLM\Software\Policies\Mozilla\Firefox\Authentication\Delegated
If this policy is enabled, the browser may delegate user authorization to the server for the specified websites. Entries in the list are formatted as mydomain.com or https://myotherdomain.com. If this policy is disabled or not configured, the browser will not delegate user authorization to the server for any websites. For more information, see https://developer.mozilla.org/en-US/docs/Mozilla/Integrated_authentication.