In a steam engine, the firebox is the area where the fuel is burned, producing heat to boil the water in the boiler. Most are somewhat box-shaped, hence the name.

In the standard steam locomotive firetube type boiler, the firebox is surrounded by water space on five sides. The underside is not so surrounded. If the engine burns solid fuel, there is a grate covering most of the bottom of the firebox, which holds the fuel; beneath the grates, an ashpan collects the solid combustion waste. Combustion air generally enters at the base, and the airflow is controlled by damper doors.

Firetubes are attached to one wall of the firebox (a side for a longitudinal boiler, the top for a vertical boiler) and carry the hot gaseous products of combustion through the boiler water, heating it, before they escape to the atmosphere.

The metal walls of the firebox are normally called sheets. The crown sheet is the top of the firebox.