To address and perhaps clarify a few questions above:
The DC-10 has a bottom-mounted external tank underneath the fuselage which is filled with a liquid retardant concentrate & water mixture (the red stuff you see being dropped over wildland fires). It is able to but will likely not carry a water or water & foam mixture only. The DC-10 airtanker is intended to operate over fires well inland or otherwise separated from suitable scooping sources, so the use of the CL
-415 waterbomber (an otherwise excellent firefighter) is inefficient in vast areas of the western US. Use of the CL
-415 in coastal areas or relatively flat terrain with many lakes remains a very sound firefighting strategy.
The Evergreen 747 had several pressurized internal tanks that carried retardant, which was delivered through an aft-mounted high-pressure nozzle aimed downwards from the aircraft. This is a similar method by which USAF
Hercules aircraft deliver retardant, and I can assure you is inconsistent at best in terms of coverage and effectiveness on the ground. Coupled with the fact the 747 drops at 800' AGL, making much of the load susceptible to drift after release, it became evident that the effectiveness did not justify the expense. In my opinion, common sense prevailed over the 'bigger is better' mentality.
Both widebodies suffer from the inability to operate from all but a handful of airports, which may be a great distance from the fires. Even if you have sufficient runway length and apron space, you need the loading equipment installed at each location. A ground turn of 12 minutes is not accurate - that's the loading time. Allow at least double that for taxiing and manoevering prior to takeoff and after landing.
Regardless of the effectiveness of each airplane type, it must be said that the key to successful aerial firefighting is in the way the aircraft are managed. The USFS
and some state land agencies still employ some grossly inefficient and wasteful tactical and strategic resource management practices that any number or type of aircraft will not overcome. But that my friends, is a topic for another site.
The nice thing about a mistake is the pleasure it gives others.