Here is how did he escape and Flash Fire goes in Ryan F-Freeman's Adventures of Transformers: Prime.
[We then see Zuko and the other members of Team Avatar on the floor of Ozai's cell]
Sokka: Ok, I am very much confused. How did he escape, and leave no trace?!
Suki: More importantly.
Ty Lee: Why would he escape, and leave Azula behind?!
Zuko: That's a good question.
Ang: Maybe I can talk to the only witness that saw what happened.
Toph: That'd be nice and all Twinkle Toes, but the guard is dead, and two prisoners escaped.
Katara: Uh, Toph, I think he meant in the Spirit World.
Mai: Well what are you waiting for, contact him.
[he then projected his spirit into the Spirit World]
Guard: Avatar, what are you doing here?
Aang: What happened in the prison? How did Ozai escape?
Guard: I can't explain it, but me and the prisoners saw what looked like a hooded figure.
Aang: But Zuko would never free his own father.
Guard: Yes, but he can't use dust to kill either. That's what the figure did. I was on the verge of death, so I couldn't stop the prisoners from escaping. The last thing I saw was that this figure somehow restored his bending, and even turned his skin red. I've seen a lot of benders with unique abilities. I've never seen anyone capable of doing what this fella did.
[shocked by what he had just heard, he left the Spirit World]
Toph: So what did he tell you?
Aang: Some hooded figure used dust to kill him, broke Ozai free, and restored his bending, at the cost of turning his skin red.
(while that sunk in, Flash Fire was taking a nap)
(he then woke up and saw two people standing there)
Flash Fire: Who are you and why are you here?
Apocalypse: You are all my children. And you're lost, because you follow blind leaders. But I am here now.(removes his hood) No more false gods. I am here for you.
Flash Fire: Ok. What is it you want?
(Nur walks to him)
Apocalypse: I have seen what you are capable of Flash Fire. I am allowing you to join me, and together, we will recreate the world.
Flash Fire: Just name what I have to do.
Flash Fire: What are you doing?
Apocalypse: Giving you new abilities, child.(turns his hair longer and into fire, and his left sword into fire as well]
[Ozai the looks surprised]
Apocalypse: Go on. Test your fiery blade on him.(points to Ozai)
Flash Fire:(does so)
(when the smoke clears, Ozai is not affected, as if the fire never touched him at all)
Flash Fire: Cool.
Apocalypse: Come along. We have one more to find before we go for the most powerful. His telekinesis blocks his mind, so we will need the third to find him.
(they all leave and the screen goes black)