Fireball Cannons are a type of obstacle attached to walls of some levels. They shoot fireballs in a straight line. Upon contact, they immediately damage the playable character.
Sonic the Hedgehog (1991)
The Fireball Cannons first appeared in Sonic the Hedgehog (1991). In this game, the Fireball Cannons shoot fireballs every three seconds. They are first encountered in Marble Zone Act 2 and later in Star Light Zone, albeit with a different appearance but same function.
Sonic & Knuckles
In Sonic & Knuckles, the Fireball Cannons appeared in Lava Reef Zone where they serve the same function to launch fireballs in the air, damaging the player on contact. However, if the player has the Flame Shield he/she will not be damaged.
Appearances in Other Media
Adventures of Sonic the Hedgehog
A Fireball Cannon appeared in the episode "The Last Resort" where Scratch and Grounder used it as a trap for Sonic but it failed and hit the volcano, which made a rock fall on them.