B747-400 Cargo Fire alerted at cruise but the crew determined it was a false warning. Before landing the Captain declared an emergency as a precaution.

Narrative

I was summoned to the cockpit because of an intermittent main deck cargo fire indication. The event consisted of the warbler; the Master Warning and a light in the overhead 'ARM' switch. No EICAS message illuminated. We; as a crew referred to the QRH; however; this item had been previously written up on several legs before this flight. Also of note: several smoke detectors where on deferral. An inspection of the main deck revealed no sign of smoke; heat or flames. There was no declared HAZMAT on board the aircraft. An inspection of the cargo documents revealed no additional items of interest. I contacted Dispatch. The conference included Dispatch and Maintenance. All involved in the discussion concurred that the problem was a spurious signal. Main deck inspections occurred at close intervals for the remainder of the flight. NO smoke or fire was ever reported. NO EICAS message illuminated. Also during this time period several items in the aft bulk cargo failed. IE: heat valve; fans; etc. Continuing to our destination we declared an emergency and upon landing requested and exterior inspection from the fire department. No anomalies were found. Again we inspected the main deck and found no trace of smoke or fire. We continued to the gate without further issues.