An adjustable firmness seat suspension includes a seat frame, first and second spring members, and an intermediate tensioning member. The seat frame includes a first frame member and a second frame member spaced apart from and opposite one another. The first spring member is attached to the first frame member, the second spring member is attached to the second frame member, and the intermediate tensioning member is attached to and extends between the first spring member and the second spring member. The intermediate tensioning member is formed from a shape memory alloy (SMA) having a first length that provides a first spring tension of the seat suspension through the first and second spring members, and configured to alter its length to a second length that provides a second spring tension of the seat suspension through the first and second spring members in response to application of a tensioning signal thereto.