She has this demonic mattress she brought with her from Japan... It goes on the floor and is the stiffest mattress I've ever felt. The first time I saw it I plopped down on it like a regular mattress and actually hurt my tail bone. I honestly woke up with bruises from it after a few nights.
It looks like a regular 12" mattress but it feels more like plywood with a few layers of fabric on it. It has a tiny bit more give than plywood but seriously not too much.
I do think firm mattresses are better but there
is a limit.