Where we live (Andes Mtns. north of Quito) there are no firebricks - only low quality red bricks. Locals build and use brick ovens for cooking so have much experience. We are told that for the cooking surface they want to make a layer comprised (as far as I can tell) of local clay, crushed glass, and lava rock. Over brick domes they use mud (unknown clay content). Does this make sense to you for pizza oven? Especially the former.