The difference between firm vs. medium mattress firmness is that a medium mattress is more on the plush side while a firm mattress is on the support side. A medium mattress is not to be confused with a medium-firm mattress, which is the middle point between a firm mattress and a medium mattress.  It is best to check how both mattresses feel when lying down on them to know the difference. But if that’s not possible or you’re buying the mattress online, looking at the mattress material is another good way to gauge its plushness. Memory foam mattresses are more likely used for medium and soft mattresses, while hybrid mattresses are commonly found on firm mattresses. For example, Purple’s memory foam and hybrid mattresses are made of two different materials that can go from soft to firm.