Firm doesn't necessarily mean that the mattress is good for your body and your back. Too often, people choose a firm mattress because they think it will help them, regardless of whether or not they find it comfortable. Choose something which you find comfortable, and if this means spending half an hour lying down in a showroom, then go for it and take your time. Just to give you an idea, people weighing between 130 and 230 pounds on average do best with a medium-firm mattress that falls in the 4-6 range on the standard firmness scale.
One of the reasons people don't like shopping for a new mattress is because it can be so difficult to find the exact firmness for your body. If you are sharing a bed with somebody, the shopping trip for a new mattress can become a long and tedious one. Finding something that you agree on isn't going to be easy, but cashmere is suitable for most people, and comes with varying degrees of firmness.