Pizza stone or firebricks to fix uneven cooking

Bricks in the oven are an old breadmaker's trick. They do increase thermal mass, and thus temperature consistency. I've kept 3 regular, not fire, bricks on the bottom shelf of my gas oven for years, and they work well. Plus they're always handy when I need to press some eggplant. If you go with something other than firebrick, you need to be sure the things are real clay all the way through. There are modern composite 'bricks' with a clay coating over something else. Those'll likely break if you bake them. The way to tell if a brick is solid clay is to break one in half and have a look. Whether regular brick, or fire brick, the first time you heat them up, you'll want to do it slowly. Say 80°C (175°F) for a couple hours before heating above boiling. That'll minimize chances of potentially violent cracking.