A house in my neighborhood was for sale, I went to two open houses, first one; someone commented about the dirty fireplace, realtor commented some muriatic acid would remove the smoke stain. A couple of months later, I went to a meet the neighbors open house, the fire place was spotless.

Muriatic acid is dilute hydrochloric acid, so treat with care and safety especially in a partially enclosed space such as the interior of a fireplace. One trick that I used years ago on soot on the outside of my fireplace was Coca Cola. It is mildly acidic and should get off the first layer.