Funny you should ask. I just put fire bricks in the FB of my Brazos and I did a short four cook yesterday to try them out.Do any of you use fire brick in the firebox? I don’t know if it’s worth the trouble or not. I haven’t even lit a fire in mine yet since we’re moving in a week and it’s still at my in laws, but the suspense is killing me.
First, I don't use the grate in the FB. I build the fire in the bottom. For that purpose, the fire bricks worked well. They protect the bottom of my FB and also change the angles inside the box. That made for a better coal bed and how the splits lay across the coals. I like that aspect.
They also do provide insulation. As to acting as a heat sync and evening out temps, I don't know yet. It will take a few more cooks and especially, some long cooks to conclude.
But I did get a better coal bed and that's worth a lot.