Texas heat does real damage to masonry. Summers with sustained temperatures above 100 degrees, followed by winter nights that occasionally drop into the teens, create extreme thermal cycling. Mortar joints between firebox bricks expand and contract repeatedly, and over 10 to 20 years that movement cracks the mortar and eventually the bricks themselves.
A cracked firebox isn't just a cosmetic problem. The firebox is the first line of protection between an open fire and your home's framing. When joints fail, combustion gases and heat find a path. That's a fire risk, and also a carbon monoxide risk for gas appliances. Homeowners near Twin Creeks or in the Stacy Road corridor who haven't had their firebox inspected in several years are overdue.