Fireboxes wear and deteriorate from extreme heat, expansion & contraction, joint decay, water penetration, and use. Many American standard fireboxes are of poor design and lack aerodynamic and reflective qualities, hence provide mediocre performance. The good part is that usually the aforementioned maladies can be rectified without costing a fortune.
Most people don’t think about smoke chamber repair when servicing a chimney system. However, when repairing a chimney this is a repair that can come up.