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Most builders don't mortar the hearth bricks (floor). If you need to replace a bad brick or one gets chipped, how are you going to replace it if it is held fast with mortar?
Mortar tends to get between bricks no matter how hard you try to keep them clean while setting them evenly, then gaps appear to mess up the others in sequence. Guess how I know this. Just fit the bricks as best you can without mortar and have faith that they won't shift. Heat expansion and subsequent cooling cycles move the bricks (and dome) around so it is a natural event.....embrace it.
Experience has shown that a loose set floor prevents problems. An oven is always expanding, contracting, and in motion with each heat cycle. The reason the floor is laid without mortar, and the first course isn't mortared to the floor, is to allow for this thermal shifting.