Polyurethane Foam - Polyurethane foam is a popular and reliable material that does a good job at shaping and stuffing, and comes in different densities for different degrees of firmness: Soft, Medium, Firm, Extra Firm, Super Resilient (SR) and High Resiliency (HR) for the most firm. Newer foams belonging the High Resilience (HR) family offer a soft, initial feel and then firm up as more pressure is put on it...Thus yielding very comfortable and supportive seating.