dry powder is good for not creating thermal shock, which can shatter the wheel or brake components, however its got no cooling effect. once fire occurs its got to be extinguished and prevented from escalating. this is what the fire crew did, from as safe a position as possible. the foam may possibly be residue from around the vehicles pump, left in a hose, or actually produced intentionally. it makes water "greasier", more free flowing and capable of dealing with a liquid fire should a pipe or tank rupture.