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