soapstone firebrick, they retain their heat longer.

Yes. The Brochure clearly states that firebrick lining is "Key" to the higher efficiency. Additionally welded steel stoves are subject to warping if you don't utilize firebrick lining.

Brick is my favorite, but the firebox will be made of firebrick, and not ordinary building brick. Some people prefer the appearance of cut stone or field stone, but the firebox will still be firebrick (refractory brick)

Yes, but surely you see it every time you clean out the fire?

Firebricks, or refractory bricks, are made from ceramic and/ or clay. The substances in their makeup are not radioactive.