Massive air freighters, or monster fire retardant bombers. Think of how much one of these babies could drop on a wildfire!
Probably too unwieldy for that application, just passing some gas here...
Not as much as you would think. As a freighter it would be weight limited. It runs out of weight long before it runs out of volume. It has about 300,000 extra pounds of zero fuel weight compared to the 747-8F. The 747-8F can carry 140 metric tons or 154 US tons while the A380F was projected to carry 150 metric tons or 165 US tons. Sure the A380F would have more range, and carry a lttle bit more cargo, but it would have had to haul around an extra 150 tons of structure. The A380F would have required special equipment at each airport to load and unload second main deck cargo containers. It would also have a problem with weaker floors on the upper deck than is normal in cargo aircraft making it difficult to fill the space even with low density cargo. Most cargo is not so time sensitive that it needs to fly nonstop on long distance routes. UPS has a hub at Anchorage, Alaska (ANC) that is ideally located as a fuel stop between Asia and its hubs at SDF and ONT. Cargo doesn't care if it has to wait around. Also, they have a sorting facility at ANC that allows transfering cargo between aircraft.
A 747-400F or converted freighter would make a better fire fighing tanker. They can land at more airports, there are lots more airframes and parts available at good prices. Depending on modifications to the structure, it might be possible to put even more tank capacity by weight if the tanks didn't have to be completely supported by the cargo floor. Considering tankers would substitute payload for range, they could decrease the fuel onboard to increase cargo while staying under the Maximum Takeoff Weight.