Over time the muffle can develop cracks. I've been told that the cracks shown here in my kiln were caused by opening the door while the kiln is very hot. I'm guilty of this, due to opening the door while enameling. These types of cracks are not harmful. As long as the muffle isn't breaking apart in pieces, and the elements are still secure inside the muffle. As the muffle heats up it expands and the cracks close.
It’s also good to check the kiln's plug wires making sure they are not cracked or damaged. If they are seek advice from a kiln repair company.