Firebreathing is a reasonably simplistic Fire-type attack.
The user inhales a large breath of air, before exhaling a stream of fire for as long as they possibly can from their mouth. This is limited by the dual factors of the amount of oxygen in their lungs, which can be limited to their lung strength (in that it can't often last for more than thirty seconds for someone who isn't trained for lung strength) and often comes in fitful bursts, starting of extremely large and tapering down.
However, while this thin stream of fire is outclassed by it's more powerful stablemate Flamethrower, it isn't useless in any sense of the word. Unlike Flamethrower, the flames may not leave the target with a lasting burn, but due to the much thinner spread of flame, is more likely to strike the opponent in a weak spot.
Due to its light power, this technique holds a D-rank, despite the increased chance of striking a weak spot.