When he and Aqua were born on Yoshi Island, both were somewhat outcasts. He was born with wings, as well as the ability to breath fire. Not having control over his fire breathing at such a young age, he had to be kept under constant surveillance to make sure he didn't burn anything important down. As soon as he was old enough to do so, he left the island, vowing never to return. He's returned every year since for the annual Spear Guy hunt.
Super Mushroom Kingdom Wars
He did absolutely nothing before getting killed by the X-Nauts during my disappearance. Feh.
(New Super Mushroom Kingdom Wars ended before Firenze was able to join…I think)
Super Mushroom kindom Wars Deluxe
He and Aqua are co-generals of the Yoshi Destroyers, since I didn't want one outranking the other. They'll also soon be co-champions. Oh, and did I mention that he's a werewolf?