Leave it alone. If she was at this company for 30 years then she understood appropriate business behaviors. You don’t know why she was fired. Maybe she used your birthday to cover embezzling money (e.g. she buys you a $20 cake but charges in $100). Maybe she yelled at someone in a check out line while wearing her work badge. Maybe she was called into HR for playing favorites and told them to fuck off. You don’t know.
I don’t know why you quit – you are not responsible for your supervisor’s behavior.