A little more detail on retardant v. foam

I asked TL Stein about the differences between retardants and foams a while back, this is what he said:

RETARDANT:  Retardant is a combination of base elements designed to slow a fire down, not put it out, by nature of chemical composition.  By design and purpose, retardant (I'll use Phos Chek XA as an example), mixed in it's various degrees, consists of a gelatin, fertilizer, iron oxide for color, water, and a few other catalysts.  When mixed, the consistency is that of a thick pudding.  It's job is to coat the fuel to shield it from the fire.  While shielding the fuel, gasses are given off when heated that inhibit the oxygen from the fuel.  The iron oxide provides a color mark (red) as an indicator of where the drop was made and the gelatin is what goops the load, enabling it to stick to the fuel.  Ammonium phosphate acts as a fertilizer to promote new growth immediately after the fire, thus enabling the burn area to recover quicker and prevent possible erosion.  Drop altitude and airspeed are all factors in the effectiveness of a retardant drop.  As retardant is released from the aircraft, it breaks up by nature. 

If a drop is made too high, the retardant ends up becoming a fine mist by the time it reaches the ground, which reduces it's effectiveness.  If a drop is made too low, it doesn't have a chance to break up properly and usually creates a heavy coating on the fuel, destroying the watershed fuel by flattening it, and could kill any firefighter caught in the drop path.  Low drops will also destroy equipment in the path of the drop and is dangerous for the aircraft and pilot, as wind shear at low altitude can and will cause and aircraft to simply run out of air, stall and crash.  If a drop is made too fast, the retardant will start to break down due to the increased airspeed and become a mist, just over a larger area.  There is a set criteria for air tankers and how drops are to be made.  Airspeed and altitude are now figured by computer, onboard most tankers.  The tank doors open to adjust to these two factors providing with the ideal amount of release of retardant in relation to the aircraft performance. Keep in mind, the computer does not control the aircraft, rather, it reads the altitude and airspeed and adjusts the retardant drop accordingly.
 
FOAM: By nature, foam physically smothers a fire.  While not having all the same properties of a retardant, some foam products do have a minimal retardant capability.  Foam allows a deeper penetration on certain fuel types and it's effectiveness dropped from an aircraft varies on the fuel type involved and fire behavior. While never having used any foam product in my working experiences (other than training for aircraft crashes and fuel fires), my working knowledge of foam is limited.  I do know that foam is a good pre-treatment for brush as well as structural protection from spotting ahead of the fire.  Even if the foam dissipates after application, it's effectiveness is still valid for a short period of time, however, not as long as a retardant.

Glossary (Fire terminology from National Wildfire Coordinating Group)

spotting: Behavior of a fire producing sparks or embers that are carried by the wind and start new fires beyond the zone of direct ignition by the main fire.