Do your mattress sink if you sit on the edge of the bed?

We have an old, very old, Stearns & Foster "Easton" and are looking to replace it. We went to try mattresses at a major high street store. We found that some mattresses when we sat on the edge of the bed just completely squished (for lack of a better word). And when you went to get up from the bed, or if you got to close to the edge, you pretty much just rolled out. It might have been the Vispring beds with a sprung divan that did this, but to be honest, we tried so many beds it's hard to recall. In any event, do your beds have a firm edge? Many thanks in advance.