From the InciWeb site (see blogroll at right), a news release about the ingredients of the retardant solutions ("slurry") dropped on wildland fires.
Aerial drops of retardant have come a long way since the first recorded water drop in 1930, when a Ford Tri-Motor airplane used a wooden beer keg filled with water. Now air tankers can drop 500 to 2000 gallons of retardant at a time to help suppress fires. Helitankers, which are helicopters with built-in tanks, can drop up to 2000 gallons; retrofitted DC-10s have an 11,000 gallon capacity, and Boeing 747s 20,000 gallons.
In the Wet Mountains, we had one of Zeus's slurry drops today: a mixture of rain and snow that should reduce the fire danger pretty well for a while.
(Via Firefighter Blog.)