If you do some cursory research into fireplace design, you're likely to realize that some kind of special mortar is recommended for firebrick in the firebox. It's natural to assume that something other than the usual masonry mortar might be required for such a specialized application. If you consult several sources, however, you may find a confusing variety of recommendations. Even the model building codes are unclear on the subject. According to Jim Buckley, a Seattle-based designer and builder of Rumford-style fireplaces, most masonry fireboxes, unfortunately, are laid in ordinary portland cement mortar, sometimes with a little extra cement in it or with some fireclay added to make it "fireclay mortar" (one common recommendation). Because few people use their fireplaces as a primary heat source (or even light fires in them very much), portland cement mortars often will work fine, at least in the short run. But after repeated cycles of exposure to extreme heat followed by cooling, portland cement mortars will deteriorate. In order to ensure good long-term performance of the installation, it's best to use a "refractory" mortar for both the firebrick lining the firebox and the clay flue lining in the chimney. Refractory mortars offer the same resistance to high temperatures as the firebrick and flue tiles.