Funny that this comes up today. As of Friday, I had
no idea that they've been using heavies for fire-fighting purposes. After that amazing discovery, I spent the next 2 hours Youtubing every video I could of the DC10 and 747 (

) dropping retardant. What an friggin' incredible job that must be.
And yes... some serious genius went into making this possible. To answer the question regarding the altitude, they have to fly low, like all other retardant planes, because the material dropped would become far too dispersed to be effective at all. It would just completely dissipate before it ever touched the ground.
One source I read (it was a quote from a fireman on the ground) said that a single pass by the DC10 over one particular fire made more of an impact than an entire day of combined helo and S2 drops. 12,000 gallons per carry, which is usually dispersed over four passes. That's a hell of a lot of material.
There was one incident fairly early on in the DC10's fire fighting career. In a turn from base to final (I presume they essentially make the same sort of downwind/base/final approach to a drop that they would make for landing... if you watch the video, with the exception of landing gear it looks like the aircraft is even configured for landing), the aircraft experienced severe turbulence, dropped a few hundred feet and the left wing clipped the tops of some trees.

After climbing out and dropping retardant, it went home for repairs. Was flying again in a few months.
To my knowledge, there are currently two DC10's and one 747 (Evergreen). If you thought watching the 10's dropping 12,000 gallons was amazing, you should check out the 747 dropping
24,000.