In the episode Zuko Alone it states that:
Since Ozai wanted to be firelord instead of Iroh, for Iroh lost his first and only son in the war, firelord Azulon was outraged. Therefor he said he wanted Ozai to kill his son, Zuko, so that he could suffer the burden of loosing a firstborn. IT GETS BETTER. Ursa, in overhearing Azula saying to Zuko, : “Dads going to kill you, dads going to kill you!….Really he is…” Confronted Ozai. He told her his plans. Seeing as Zuko was Ursa’s favorite, she probably freaked out. SOOOO…she gave Ozai an idea. THIS IS WHAT HAPPENED TO HER…she basically told Ozai that he could become firelord by letting Zuko live. She gave up herself…how? She told Ozai that she would murder firelord Azulon, that way she would be banished forever and Zuko would live. It was all part of the plan. So she was alive…in the end.
weird huh?!