A jacketed firetube system for use in a process vessel such as a heater or heater/treater has a jacket which extends along the firetube, at a flame inlet end. The jacket can be external to the firetube or internal to the firetube. A heat transfer fluid is circulated along the firetube between the jacket and the firetube for recovering heat from the firetube and reducing the firetube's temperature. The recovered heat is reintroduced into the system.