When performed, Mario would take out and absorb the power of a Fire Flower, transforming him into Fire Mario. This would infuse his attacks with fire, thus making him stronger and faster. Fire Mario was one of the many Final Forms in the game, which meant that he had a different and more powerful moveset than normal Mario.
Fire Mario is a form in the Mario series that Mario takes after obtaining a Fire Flower. Originating in the original Super Mario Bros., this form is the most common of all of Mario's powered-up forms throughout the series, appearing in almost every Mario game featuring power-ups. It most often recolors Mario's clothing to white with red overalls and gives him the power to shoot fireballs. Contrary to its use in SSF2, the form generally has little other use beyond this, though it does grant Mario strength, speed, and even flight in The Super Mario Bros. Super Show!.
Fire Mario's early portrait art, used from v0.5a to v0.8b.
Prior to v0.5a of the SSF2 Demo, Fire Mario used a completely different set of sprites and a much different moveset. His standard attack was a seven-hit beat-down and all of his special moves were completely different. The Super Fireball did not release the Mario Finale at full charge (though he released a mini-version of the attack), his side special move was Heat Shield rather than Golden Cape, and his down special move was Fire Mario Eruption rather than Firewall Tornado.
Out of all of the removed content throughout the SSF2 demos, Fire Mario has proven to be the most controversial. Fans commonly request its return, complaining that its absence detracts from SSF2's individual identity and makes Mario too much like a clone of his official Super Smash Bros. appearances. Despite these complaints, the developers have stated that it will not be returning.