Abstract: A boiler firebox comprises arcuate front and rear end water walls connected respectively to front and rear ends of right and left side water walls. The end walls curve outward from respective ends of one side water wall and then inward to the respective ends of the other side water wall. The end and side water walls enclose a combustion chamber, and are connected at ends thereof such that water can circulate through the end and side water walls. Water input ports can be provided in water walls on opposite sides or ends of the firebox such that the flow of circulating water can be divided with about half entering the water wall on one side or end of the boiler and the other half entering on an opposite side or end, thereby reducing temperature variations in water near the input ports compared to boilers with a single input port.