Firmness control for a self-adjusting, non-powered smart response technology mattress includes a body support device; one or more layers overlaying the body support device and a side rail assembly circumscribing a perimeter of the body support device; and the firmness control. The body support device includes a plurality of fluid support cells, wherein each fluid support cell includes an envelope and a reforming element disposed within the envelope; and a non-powered manifold system including a manifold conduit fluidly coupled to at least two of the fluid support cells, and intake and exhaust valves fluidly coupled to the manifold conduit configured to dynamically open and close in response to a weight load. The firmness control includes a pressure relief valve fluidly connected to the manifold conduit via a conduit and configured to selectively permit a controlled amount of fluid flow out from the fluid support cells. Also disclosed are processes for adjusting firmness.