Type A firefighting foam is a foam used for fire suppression. Its role is to cool the fire and to coat the fuel, resulting in suppression of the fire.
Type A foam comes in buckets that can bet retrieved from the foam cabinet inside the fire department (The foam cabinet comes with the fire trucks, if your FD does not have one ask the owner of the trucks about it). The fire trucks themselves have an internal reservoir of foam that is used when putting out fires, the capacity varies from truck to truck. To fill a truck with foam, place the bucket on the ground next to the truck and click it. Select the truck you want to be filled with foam and it should fill it to its capacity, and display what is left in the bucket.
Fire Fighting Benefit
The benefit of using foam over water when fighting fires is foam is more effective at suppressing fires (They spread less) and and stuns the fire so it cannot grow for a brief period.
Since trucks only carry a limited amount of foam caution should be used to avoid wasting it.