Furniture stores, not surprisingly, tend to range between the chain stores and department stores and the smaller sleep stores in terms of their mattress knowledge, the value they offer, and the knowledge and service they provide. Some of them are closer to department stores in that their mattresses are only secondary to the main purpose of their business and this department is staffed by people who will likely know less than you do about mattresses. Some are much closer to smaller specialty outlets and sleep shops in the service and value they represent and sell quality furniture right beside a separate mattress and bedding section staffed by knowledgeable people and offering both the more common "S" brands right beside the "off brands" that have higher quality and value. The best of these can be well compared with their more specialized, smaller counterparts and are really a sleep store inside a furniture store. these cased they represent a good source for value, are part of the "resistance movement" in their approach to business and service, and well worth your time and business.
The other half of this group tends to be closer to the department stores, have more poorly trained staff, and use similar sales techniques and methods. The comments relating to department stores and chain stores would be more applicable for the furniture stores that fall into this category. With a little bit of education on this site or a few questions in the forum you will quickly be able to tell the difference and discover which of these are worth more time in your mattress search and which should be used for mattress testing only.
The best of these will offer high quality mattresses at fair prices.