there are many different types of fire brick. The typical hard yellow grainy stuff you pick up at the hardware store is firebrick made for fireplaces, not at all a refractory firebrick.
Different firebrick is rated for different temperatures. Soft firebrick, and a lot of hte hard firebrick is rated for upwards of 3000 F
That being said, the forge in that picture is not your typical propane forge made from firebrick, tha'ts a charcoal/coal burning bottom air-fed forge which functions entierly different