While holding the B button, Bowser continuously exhales a stream of fire in front of him, which gradually decreases in length over time. In Brawl onwards, the flames can be angled up and down and will never completely recede, unlike in Melee, where Bowser will eventually begin to exhale smoke instead.
In general, Fire Breath's long duration is useful for staving off opponents and preventing their return to the stage, while also assisting with defensive play in neutral while dealing heavy damage. Unfortunately, an opponent using DI can escape the flames fairly easily in most directions, although as time has gone on this issue has become much less prevalent. For example, in Ultimate, opponents can rarely escape without taking an upwards of 25%. If used too much, however, Bowser will have to recharge his breath, punishing players for overuse and requiring it to be used carefully. The fire will flow off of floors and walls, and can actually split in two directions if the angle is perpendicular, allowing for interesting mixups on walled stages.
In Melee, Fire Breath originated as a defensive maneuver for Bowser to make use of. However, its slow startup made it a very risky move to use, with it requiring 28 frames just to start coming out. However, it greatly made up for this in its formidable range and large amounts of hitboxes. It was easy to SDI, however, making it overall one of the worst projectiles in the game. This was due to characters being able to SDI towards Bowser, escape it, and punish Bowser for virtually free due to the 30 frames of endlag. Flame canceling helped the move and gave it utility, but was removed after NTSC 1.1, leaving it out of the tournament standard. Thus, as a result of it being easy to react to and its endlag, Bowser's Fire Breath was solidified as one of the worst special moves in the game.
Brawl enhanced the move's utility, making it more centered around being a neutral tool than in Melee. It was much faster as well. The ability to angle the move has also improved its versatility as well, furthering its use as a neutral tool. This allowed Bowser to have more options against aerial approaches, rather than having to commit to stop them. It was also made into a formidable tool at the ledge, with characters like Donkey Kong having no chance of recovering if the move was used in the correct spot. However, Fire Breath's horizontal range was reduced, making it slightly more difficult to use it defensively.
In Super Smash Bros. 4, Fire Breath gained improved speed and reach, but the outermost hitboxes could no longer flinch foes. However, the buffs greatly outweighed the nerfs, with it being combined with his formidable grab game and forward aerial to create a tough selection of attacks for an opponent to escape. Players could simply stop Fire Breath early to prevent the non-flinching area from ever coming up, and react to the opponent's response accordingly.
Ultimate dramatically increased the damage output of the move, which, when combined with the increased 1.2x damage in 1v1s, allows it to consistently deal around 30% at close range before running out, making it an extremely potent punish and a mainstay in his defensive toolkit. Opponents are more inclined to panic due to this drastic increase in damage, allowing Bowser to bait out worse options and punish them harder than before.
This special attack is shared by Giga Bowser, both in his boss appearance in Melee and in his playable appearance in Brawl and Smash 4. There is no time limit to how long the attack can be held in his Melee or Ultimate incarnations.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Bowser's ability to breathe fire has been demonstrated in many games he's appeared in, including his debut in Super Mario Bros., where he would shoot fireballs at Mario. These fireballs are also the inspiration for Fire Shot, one of his customizable moves in Super Smash Bros. 4. However, it wasn't until Super Mario 64 and many games after that he was shown to breathe continuous streams of fire.