A Fire Cannon is an obstacle that first appears in Super Mario Galaxy.
Super Mario Galaxy
They appear in Bowser's Star Reactor, Deep Dark Galaxy, Toy Time Galaxy, and in Undergrunt Gunner Planet. There are two variants of this obstacles. One that emits flames, and the other shoots fireballs.
Super Mario Galaxy 2
They reappear in this game, in Bowser's Galaxy Generator. Notably, the game only features those which shoot out fireballs.