Hi, I believe I have nearly the same fireplace in my house. the firebox developed vertical cracks in it. I went to the fireplace store and they sent me to the local hardware store. There I obtained a container of Frunace cement made by Chimney Sweep. It cost all of $2.00. I followed the directions on the container and it worked like a charm!!! I have had many fires in the box sence and the cement it holding perfectly! One tip. I used putty knife to spakle the crack and a damp rag to smooth the product in place.
Hope this helps. Good luck!