Foam density can be considered a measure of quality. The most important factor that will determine the lasting comfort, support, and durability of foam is directly related to the density, which is measured by weight per volume. This is expressed as lbs per cu ft.
The higher density of the foam the more it costs to produce. Higher density foams have more lasting resilience (springiness) and are less likely to lose their shape in usage.
It is important to know that density is independent of firmness. Extra firm does not necessarily mean high density. Firmness is a measure of load bearing capacity, which is also called the Support or Compression factor. A high density/firm cushion will keep its original feel far longer than a low density (lower quality)/firm cushion.
The firmness type chosen is a personal comfort preference, which can also vary with the thickness of cushion and the type of furniture. If you have an inexpensive sofa that is uncomfortable, a higher grade seat foam will make a world of difference to your comfort and extend the life of your sofa.