B747 pilot reported a fire warning prior to the top of descent and after visually confirming no fire; landed normally. Reporter suggested the low temperature in cargo area produced fog which the smoke detectors falsely identified as fire.

Narrative

About 150 miles from ZZZZ we got a Main deck aft" fire warning and bell. We knew we were loaded with flowers and we had talked about the possibility of this happening during the preflight briefing. We were a crew of 4; so one of the extra crew members went down and did a thorough check; seeing and smelling no smoke or evidence of any smoke or fire. We elected not to run the fire checklist and proceed to ZZZZ and not notify ATC. As soon as the fire bell went off I immediately turned the temperature up on all zones in the lower and main decks to increase the temperature-dew point spread and dissipate the fog I assumed was causing the smoke detector to activate. Within 5 or six minutes the fire warning disappeared. As we were approaching the top of descent the ACM (Additional Crew Member) went down to do another inspection. Again; no smoke or fire. We landed without incident. We all; as a crew; agreed that this was the best and safest thing to do. Cause: Flowers are a high moisture low temperature item to ship and subject to fogging the air once it is full cooled. Clearly our smoke detectors are not equipped to carry this type of cargo at these temperatures. Suggestion: My suggestion is to take this issue seriously. Either fix the detectors so they are not susceptible to this; and/or not carry flowers until this issue is resolved. This event; given the right situation; could prove to be dangerous or deadly. In the event of this being in a lower lobe and not being able to verify the lack of a fire could end up with a forced off airport landing or ditching. We all know how long the survival time is with a real cargo fire; we've seen it. If we can not verify the lack of a fire; we have to assume it is real. To think that this is becoming routine is terrifying. This is my second experience with this."