Hi dzignr_tastz,
With latex density is directly related to firmness (but only if you are comparing the same type and blend of latex).
With polyfoam and memory foam density is the most important factor in the durability of the material (see post #4 here and the posts it links to about the variables that can affect the durability and useful life of a mattress relative to a specific person) but any density of memory foam or polyfoam can be made in a wide range of firmness levels. Density does have more of a connection to the compression modulus of the foam which is also part of perceived firmness (see post #4 here) but it’s not directly related to comfort or firmness. There is also more about some (but not all) of the different foam specifications that can affect the feel or suitability of a mattress design in post #2 here. Resilience is a measure of the percentage rebound when you drop a steel ball on a layer of foam. Memory foam for example has almost no resilience regardless of its density because it’s a slow response material that “absorbs” energy (and turns it into heat) while latex stores energy (like an innerspring) and then bounces back and has a higher resilience than most other types of foam which is why it’s more “springy” or “bouncy”.
Phoenix