Billy HW, Its sounds like you might have been thinking along the same lines as me with regard to Flight MH370.
This describes fire prevention methods on the Boeing 777.
Fire Protection: Passenger Cabin
I can't find any MD-11 specific info, but this gives a general decription of requirements:
Rule change: Cargo-compartment Smoke detection & fire Suppression
I did wonder if the MH370 pilot might have depressurised the aircraft as a last ditch measure against an electrical fire.
Another hypothesis would be a fault in the inerting system, in which a leak in either the nitrogen-enriched air pipe dropped the oxygen level in the aircraft, or a leak in the oxygen-enriched air waste pipe from the air separator increased the local oxygen content to a dangerous level which caused a fire.