Concerning Ursa, I think she poisoned Fire Lord Azulon. Then Fire Lord Ozai banished her for her own safety maybe? I think that’s what happened. Especially since Fire Lord Azulon was going to have Ozai kill his first born, and Ursa was VERY protective of Zuko. So her love for Zuko than her fear of death or banishment and so she killed Azulon, but made it look natural.
Also, I’ve been wondering if Ursa was a firebender. Everyone in the royal family is, except maybe for Lu Ten (Iroh’s son), but that’s unconfirmed. It Would seem a little strange if she was the only nonbender in the royal family, I mean her grandfather WAS Avatar Roku.
“Yes! We defeated you for all time! You will never rise from the ashes of your shame and humiliation!… Ah that was fun.”
~Azula, after a game of volleyball on Ember Island.