When the rear firebrick break, do they just crack and stay in place?? or do they break and fall into the fire, leaving the steel exposed to the fire? If the brick just cracks and stays in place, don't worry about it. the crack will not allow the fire to get to the steel, just like the gaps or divisions between the bricks, there is no harm or danger unless you have the hot coal fire right up against the steel box.
The cracks could be because of a movement or pressure created by the shaker grates, have you checked this to see if it is hitting the bricks?
Hope this helps.