I don't think that stars are technically made of "fire" in our sense of it. In the end really it's kind of irrelevant, as somone said of this subjct on a different thread, when the fireball hits, you're not really going to care what colour it was.

in reality no

Ozai's lightning is green/white, and is absolutely gigantic when compared to anything Azula or Iroh could do. And look again at the fireball Aang blocks in the NYCC trailer. We have never seen a fireball before that has that much pure white in it. All of the other firebending in the series has been pure orang/red/yellow (or blue/white for Azula). Even if it is technically not white fire, this speaks to me of the fireball being pretty much the most intense attack we have seen from a firebender so far (plus, as I said before, it matches the white fire made by the dragons).

In any case, Ozai does not strike me as the type to know any unique moves of his own. he strikes me rather as the type to have mastered all of the well known firebending moves to a degree beyond any other firebender. His lightning is an example of this: the ability to generate it is not unique to Ozai, but nobody can do it anywhere near as well as he can.

If Ozai were to demonstrate a unique move however, I know what I would like it to be: lava bending. Thus far we have only seen lava bending a couple of times from various avatars. And judging from the massive amounts of lava that we glimpsed in the trailer, who knows? Ozai may very well lava bend.