Hi KevinTMC,
Thanks for taking the time to share your comments and feedback about your new mattress … I appreciate it 
[quote]The new mattress is a little less firm on the edge. If the old mattress had grown concave, this mattress sometimes feels like it might be a little convex (especially when I lay down on my back, and especially on my side of the bed). It certainly feels rather different, and gives quite a bit more, when I sit down on the edge to put on my socks in the morning. I"m not sure if the slight narrowness of the frame has anything to do with that.
I’m figuring a lot of this may just be striking me because this is our first foam bed. I remember that when we were looking at Tempur-Pedics in the store, as much as I loved laying on the Rhapsody Breeze, it concerned me that when I sat down on the edge it felt like I was going to slide off. My new Tuft & Needle is certainly firmer than that…and even if it is a little convex or softer at the edge I’m sure I’ll easily get used to it. I just have a tiny worry in the back of my mind about this getting much more significant (maybe not long after the 60-day trial period expires). Is there a good way to tell whether I’m fretting over nothing, short of having an expert take a look at the bed in person somehow? Is there something I should consider adding to, or tweaking about, the bed setup?
I might rotate the mattress when we change the sheets this weekend, and that would at least tell me whether it’s my imagination that the edge softness is more significant in one quadrant of the mattress than others. [/quote]
Unlike many innerspring mattresses … most foam mattresses don’t have (or need) edge support as long as the firmness of the support core and the thickness of the comfort layers are suitable for your body weight but all foam mattresses will feel a little softer and compress more deeply if you sleep with most of your weight on the very outside edges of the mattress or if you sit on the edge of the mattress because your weight is more concentrated when you are sitting than when you are sleeping on the mattress (see post #33 here). There is an “edge effect” with foam materials because if the outside edges are compressed under weight they are only being “held back” by the material on one side of where you are sitting or sleeping and not the other where there is no material beside the point of compression to distribute the load more evenly.
The convex feeling you are experiencing is also part of adjusting to a new mattress because your frame of reference and what you are used to is a sagging mattress that dips so a flat mattress may feel like it is convex in comparison to what you are used to.
It’s unlikely that your foundation would be causing the lack of edge support although you can test this by putting the mattress on the floor to see if it makes any difference when you are sleeping on the edges. You can also sit a little bit further back when you sit on the side of the mattress to help distribute your weight more evenly and your weight would be more towards the center of the mattress so you would sink in a little less.
In most cases and for most people this is just a matter of getting used to the “feel” of a mattress that is different from what they are used to but for the few people where edge support is a more significant issue and only want a foam mattress then the only real alternative is to choose a mattress that is specifically designed to have firmer edges.
Phoenix