Do firm mattresses hurt your back?

I have a memory foam firm mattress and it doesn’t hurt my back. Now a hard mattress will but this one has enough give that it is comfortable.

got a Purple Hybrid. i absolutely love it. was top pick for fibro. it's like a memory foam bed except it stays much cooler. medium firmness

No, soft mattresses do. I use an extra firm mattress.

Extra firm for me. If it's foam, it'll be regular firm fast enough. Soft mattresses don't provide support.

I think they help to a degree. Depends on the firmness - and your back I suppose.