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'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.