A Fire Cannon is an obstacle that first appears in Super Mario Galaxy. They resemble the Flash Liquidizer Ultra Dousing Device, but they are red and shoot fire from their mouth. Mario cannot destroy them.
Super Mario Galaxy
Fire Cannons 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
Fire Cannons reappear in this game, in Bowser's Galaxy Generator. Notably, the game only features those which shoot out fireballs.