Fire Mario is one of Mario's Power-Ups. Mario can access this form by using a Fire Flower. The Fire Mario form is endless in most games (until a enemy manages to hit him), but it is a time-restricted form in other games. Wario's version of this is Dragon Wario.
Fire Mario can shoot Fireballs, completely the reverse of Ice Mario. This was one of his first forms. Luigi's fireballs tend to be green and defy gravity. Wario's fireballs are yellow, defy gravity, and move in a pattern similar to that of his mustache. Waluigi's fireballs are purple and bounce either high, low, or at a normal height.
Fire Mario makes several minor appearances within Jake's Super Smash Bros.. In addition to having his own trophy, Mario and Luigi have their respective Fire Mario/Luigi colour schemes as alternate costumes within the game. Mario's neutral special move also sees him throw a small Fireball, akin to how he does as Fire Mario.
The Super Mario series is well known for its large variety of different, and very interesting, power-ups. Super Mario Bros. was the first to feature one of the power-ups from Mario's ever-growing collection, and it was the Fire Flower. Touching the Fire Flower would turn Mario into Fire Mario, granting him the ability to throw small fireballs to defeat most foes. Though he doesn't need to touch a Fire Flower to gain the ability to throw fire in Super Smash Bros., Fire Mario is immortalized as one of Mario's alternate costumes and as one of his special moves.
Fire Mario's classic form was the first time Mario got a red shirt. Now, he has red pants, just like the classic Mario.
The fire Mario in Super Mario Land 2 had a feather instead of the usual fire outfit.