The Fire Kingdom is home to most of the Fire Elementals of Ooo. It is made up of volcanoes and seas of lava. Its monarchy lives in the Fire Palace. The Fire Kingdom made its first official appearance in "Thank You" and was a major location in "Incendium" and "Ignition Point." The majority of its inhabitants consider themselves to be evil, including the king.
It first appears in "Evicted!" in a flashback that shows Marceline hula-hooping in a sea of flames. It's possible that the Fire Count ruled the land in the past, a flashback in "His Hero" shows him wearing a crown in what is appears to be the kingdom. Flame King currently rules the Fire Kingdom, In "Earth & Water," he was overthrown by his daughter and Cinnamon Bun and placed in the cell lamp that he held his daughter.