The one bed purchase with a real trial behind it: for back pain, medium-firm beat firm. Beyond that, choose by fit to your body and sleep position, and a fair trial.
A mattress is a support core (coils, dense foam or latex) under comfort layers that let your shoulders and hips sink in. Its job is to keep your spine roughly in line in your usual position without pressure points. Side sleepers and lighter people usually need more give; heavier people and back or stomach sleepers usually need more support.
The main types (all-foam, latex, innerspring and hybrid) differ mostly in feel, heat, motion transfer, edge support and durability, not in anything health-related. Firmness labels aren't standardized between makers, so the return trial is the real test.
One well-run double-blind trial of 313 adults with chronic low-back pain found medium-firm mattresses reduced pain in bed and disability compared with firm ones over 90 days, and later systematic reviews of mostly small trials lean the same way. The trial didn't test soft mattresses or specific materials. There is no good evidence that any mattress type improves sleep or health in people without pain, and before-and-after 'new bed' studies can't separate the mattress from novelty.
313 adults with chronic low-back pain were randomly given a firm or a medium-firm mattress. At 90 days the medium-firm group had less pain in bed (odds ratio 2.36) and less disability (odds ratio 2.10) than the firm group.
24 controlled trials published since 2000. Mattresses described as medium-firm and custom-inflated (self-adjusted) came out best for sleep comfort, quality and spinal alignment; evidence on mattress temperature was insufficient.
39 studies up to 2019, classifying mattresses by the European firmness standard. The authors conclude that a medium-firm mattress promotes comfort, sleep quality and spinal alignment.