On new wood-burning fireplaces, first determine what you are burning. You should only be using seasoned wood, newspaper, or kindling. Firestarters can cause a smell. Seasoned fireplaces: what have you burned in the past? Have you cleaned the fireplace prior to burning? If you have used firestarters and/or prefabricated logs, these products are bound together with a wax-type substance, which does not dissipate. This wax can pool and has been found under the hearth refractory and can cause a very foul odor. Also check what is on top of the fireplace. Remove all objects from mantel to see if this eliminates the smell.