I honestly don't think that a close range fireball scar would in any way heal over time. It is just too damaging. That causes third - degree burns, and I really don't think that a fireball with that level of intensity will allow the skin to heal over time. Especially when Zuko declined when Katara asked him to heal the scar when they were trapped underground in Ba Sing Se. I feel that the scar defines him, and a symbol of his trials and tribulations over time. That's just my take on it; I didn't expect the scar to go away, and it's a nice reminder that Zuko, at age fucking 90, is still a badass.