The fire detection system draws air from the holds over 2 "loop" smoke detectors. Smoke needs to be detcted by both loops before a fire warning is annunciated.
Note this system is not using the hold temperature to detect a fire although the temperature can be viewed on the EICAS ECS page by the flight crew.
If one of the smoke detector "loops" is faulty, the MEL allows it to be effectively locked out and the aircraft dispatched normally. In this case only one detctor needs to detect the smoke to trigger a fire warning.
If both smoke detector loops are faulty or the fire extinguishers cannot be fired for any reason, then the holds must be kept empty.
At least that's how I understand it ....................................