Hi… I recently made a terrible purchase buying the simmons beautyrest hartfield luxury firm mattress from mattress firm and it’s already sinking and sagging after 3 nights! I’m small so this should not be happening . I really need a very firm mattress but can’t seem to find anything good decent at the store for an exchange (I can’t return it). Any ideas about very firm/dense mattress toppers that aren’t memory foam or don’t create that molding experience of memory foam? I’m a stomach sleeper and this mattress is killing me. Also, what do you think of the extra firm Garrison or the Bridgewater at Mattress Firm as alternatives? Bridgewater seemed like it could sag quickly too… Wish I didn’t have to make this work but since I do, I’d so appreciate your help.
Hi Carey99,
Welcome to the Mattress Forum! 
I’m sorry that you didn’t find us before making your initial purchase. I’m guessing what you’re experiencing would be a very initial loss of “false firmness” of the mattress, as even very low density polyfoam won’t lost structure in three days. You could also be experiencing settling of the SurfaceCool fiber material, as well as some settling if the top quilt panel was cut a bit too large.
Even with that being said, the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress, manufacturers/retailers, or combinations of materials or components because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP (Posture and alignment, Pressure relief, and your own Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
I would tell you to review the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones.
When considering any mattress, I would always make sure that you find out the information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
A stomach sleeper generally requires firmer comfort materials, and if your mattress is already softer than what you desire on top, placing a topper above it, even a firm latex topper, will still eventually result in a sag as these less durable lower density foams in the mattress begin to impress.
As you’re already aware, there is not much meaningful information offered about the mattresses you’re considering at Mattress Firm or from their manufacturers. In situations like this, when there are no options for a refund and you’re not able to find anything using higher quality materials, I generally recommend finding something using as strong of an innerspring as possible and the highest density foams and the firmest you can find, and something that is not overly padded, and at that point if you need extra comfort you could add something more durable like a higher quality latex topper from another source.
There is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable and knowledgeable supplier (that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market) can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success.
While I know it’s not ideal, that is the best advice I can offer in such a situation, If you are looking at anything else specifically and are able to acquire actual specifications, feel free to post them back here and I’ll do my best to comment upon them.
Phoenix