We used an organic mattress in our bassinet...it was kind of spongy, but still firm enough IMO (sat on a firm board). It was made from a sponge-like material, but if you moved your hand away, the imprint pops out again. I would never have used it if I thought it was a suffocation risk in the slightest
ETA: My description of the mattress was terrible so I googled the exact one:
Latex Breather Hole Mattress: The Seed Organic Cradle has a latex mattress which is a 100% natural organic product derived from the milky sap of the Hevea Brasiliensis (Rubber Tree). It is hypoallergenic and anti-microbial which means it does not grow bacteria or mould and nasty dust mites and bugs won’t burrow inside. It will contour to the shape of your baby’s body, building up no pressure points and is a breathable open cell structure for better air circulation, making it a far superior mattress for your baby. And what’s best is that it’s a naturally derived renewable resource – so it’s a good choice for the future of our environment.