My shoulders hurt from sleeping on a mattress that's too firm

What worked for me was a thick mattress topper. I got a 3-inch soft latex one and it made a huge difference for my shoulders. It was way cheaper than a new mattress.

Side sleeping on a firm mattress is tough for heavy people. Your shoulder gets all the pressure. Have you tried sleeping with a pillow between your arms? It takes some weight off the shoulder.

Toppers can help but they can also make you feel like you're sinking. For 260 lbs, a soft topper might bottom out. Look for one that's dense but soft on top. Maybe a wool topper? They're supposed to be good for pressure relief without being squishy.

I'm around your weight and I had the same issue. I ended up putting a 2-inch soft foam topper on top of my firm mattress. It was chea and it lasted about a year before it started to flatten. Not ideal but it got me through.

Latex toppers tend to last longer than foam. They're more expensive but you might get more mileage. I've had mine for two years and it's still holding up.

Also, check your pillow height. If your pillow is too thick or too thin, it can throw your shoulder alignment off. A thinner pillow might help when you're on Your side.

I'd be careful with memory foam toppers. They can sleep hot and for heavier people they sometimes don't provide enough support. If you go that route, make sure it's a high-density one.

Have you considered rotating or flipping your mattress? If it's a two-sided firm mattress, flipping it might not help much, but rotating it can even out the wear. Probably won't fix the shoulder pain though.