---That was also the first time Zuko actually took his uncle's advice. Uncle gives advice many times throughout the season (indeed the whole show) and this here is the first time Zuko does not do the opposite and heeds it.
In the Legend Of Korra-, an earthbender, Bolin, can bend lava. (And so can one other earthbender). This could be because Bolin is the son of an earthbender and a firebender. So lava bending is the result of a mixed bending family.