It is not uncommon to experience a crack in the firebox over the life of your grill. A crack can occur due to the stresses placed on it from the expansion and contraction from repeated heating and cooling.
The two main functions of a Primo firebox are to hold the lump charcoal and to protect the ceramic grill base from exposure to the high heat of the lit charcoal (815°C/1,500°F+).
It can be alarming to see a crack. A crack actually serves as a “relief point” for future expansion and it may not crack further. Even if the crack expands the full height of the firebox it will not affect the cooking performance and it is completely safe to use.
Firebox information from the Naked Whiz Web Site