Abstract: A firebox cover for thermally sealing off a firebox while not in use comprising a flexible barrier member, a sealing member and a magnet member. The flexible insulating barrier member covers a firebox opening within a room preventing thermal energy within the room from exiting through the firebox opening and preventing thermal energy within the firebox from entering the room. The barrier member has a cellular structure with a thermally reflective front face and thermally reflective back face. The sealing member about the periphery of the barrier member provides a substantially air tight seal about edges of the barrier member when the sealing member is magnetically affixed to a metallic surface surrounding the firebox opening during use of the cover.