Doesn't quality latex foam have pretty comparable breathability? And my impression was that the firmness of foam was widely variable based on the type and density of the foam.
On the firmess you're right, it is very variable. I've been myself sleeping on a foam-only sofa-bed mattress for almost a year and it is firm enough. The issue with latex or foam is that they distort with time. It is unavoidable. Depending on the quality of what you have bought that might be years but I've seen many poorly made mattresses that are returned within months.
It also depends on the weight of the people that is going to sleep as well. My wife would be worried if you were a customer insisting on buying a foam mattress and your weight as a couple were bigger than 180kg for example. Specially if it is a wide mattress as the weight would not be distributed evenly.