It would make sense that "lavabending" would be an avatar only ability. However it would make more sense for earthbenders to bend it than firebenders. This is because lava is a form of molten rock or earth. This would make sense, because if earthbenders can't bend lava, why should water benders be able to bend ice, water, and steam/vapor? A bender should be able to manipulate all states of an element. some people might argue that Sozin was able to bend lava. This is not true. If you watch the episode, Sozin only redirects heat from the lava. Heat redirection/control is a firebender ability, such as when Iroh heats his tea.