Hi JBR2022,
You are correct in that most manufacturers of mattresses and slatted foundations recommend no more than 2"-3" for latex, no more than 5" for polyfoam between slats. As MattressToGo stated in his expert reply … especially for foam ad latex support, any gap larger than that will allow sagging of the mattress between slats, affecting support as well as potentially damaging the mattress.
I looked at the Mattress Firm website, and though I am not seeing a particular foundation model with 8" between slats. It may be that you are looking at a metal bed frames something like their Hollywood metal bed frame which seem to have about 8" between metal “slats” (the link I provided leads to Amazon for the same product that is less expensive). Since they are a general mattress retailer, they carry products from many different manufacturers, some of which are possibly not very of high quality. When buying any mattress, the manufacturer will usually have very specific criteria for a foundation/ bed frame that their product should follow, in order to maintain any warranty or exchange policies. You should always follow these recommendations, since, as you have found, some companies offer products which do not fit the standard recommendations…though a lighter sleeper on a lighter mattress, with an innerspring support layer would likely not have any issues with this metal foundation frame. You did not say what type of mattress you are planning to place above this foundation but Phoenix’s thread about "The Best Foundations or Base for a Latex or All Foam Mattress"may be more relevant to what you are looking for. In any case if you own a latex or an all foam mattress I hope you are able to return this foundation with “8 inches between the slats”, or if not, you can potentially get a grid to put on top of the metal bar or in the case of a wooden foundation you can always add some additional slats which you can use to bring the foundation in line with recommended spacing.
~ Basilio