[dmun]

I got a bricklaying tool from Ireland, which I assume has the same standards as the UK. It was a perfect fit to US house bricks. Firebricks, I don't know.

The idea I sketched above should work with any brick. The two ends of the brick are cut off with a square inside face on the 7 degree angle, needed to form the vertical course of the dome. On the first layer, the piece left over in the center of the brick is laid horizontally to form the outside circle. These are good as cut for the first layer or two of the dome, then they need to be trimmed at an angle as the courses tilt in.