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Hi elapid,
With polyfoam and memory foam IFD is a comfort spec not a quality spec while foam density is a quality/durability spec not a comfort spec. Having said that … the IFD of memory foam isn’t comparable to other types of foam because the IFD can vary with temperature, humidity, and the length of time it is compressed and the testing protocol that is used to determine IFD produces different results with slow response materials than with fast response materials (an 18 ILD memory foam would feel very firm to most people for example while if polyfoam or latex was 18 IFD then it would feel very soft).
With latex … the “quality” specs are the type and blend while density is a comfort spec (firmer versions of the same type and blend will be denser).
Just to keep things a little more simple … there is more about the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here AND the other posts it links to which should answer most of your questions. Firmness is a secondary factor in durability but density is the primary factor with polyfoam and memory foam.
There is also more about the many different types and formulations of memory foam in post #9 here and in post #8 here but all memory foam is an open celled foam and this only differs in degree.
Hopefully these will aswer all the questions you asked 
Phoenix
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Hi elapid95,
I think you may be misunderstanding what he is saying because he is well aware that density is an important durability spec. One of the things we have talked about most frequently on our many phone conversations is “bemoaning” the general lowering of foam quality/densities in mainstream mattresses.
Not surprisingly … this is all correct. Any density of polyfoam can be made in any firmness level and this is also true to a lesser extant with memory foam as well (although all of it would be somewhere in the “soft” range) so foam density alone doesn’t determine the firmness of the foam (both low density and high density foams are made in a wide range of firmness levels) and it isn’t the only factor that determines some of the other properties of a foam material as well.
In most cases on this site … “quality” is used in the context of durability (which is why I so often write quality/density or quality/durability) because it’s the issue that most people are most concerned with but there are also other definitions of the “quality” of a foam that are relative to some of its other properties or specs or that determine its cost such as its resilience, surface feel or hand feel, point elasticity, how open celled and breathable it is, its compression modulus, its tensile strength, and many others (see post #4 here) but these aren’t directly connected to the durability of a foam material and are more connected to how it “feels” and performs in a particular application or layer in a mattress.
Phoenix
Note Added Later: To elapid95
I got your message…No problem! I will proceed with deleting your old posts as per your request.
Phoenix