Foam Fire Extinguishers are best for fire involving solids and burning liquids (Class B fires).  Foam extinguishers are great at cooling fires, the spray of the foam helps break the connection between the flames and the fuels surface.

How to use Foam Fire Extinguishers
For solid fires point the nozzle at the base of the flames and sweep across the area of the fire.
For liquid fires, point the nozzle at the rear of the container and allow the foam to build up and flow across the flames. Fully discharge the extinguisher to maximise the cooling effects. Warning, do not use water extinguishers on burning fat oil or any liquids.

Advantages
• Smothers the fire by creating a film on top of the fire.
• Cools the fire through evaporation of the water content in the foam.
• Excellent for burning liquids like petrol.
• Foam extinguishers are safe for use with electrical equipment if they had a successful dielectric test.

Disadvantages
• Not environmentally friendly.
• Not effective if the fuel is flowing in a stream.