I call my MIL Mrs. Stephanie (her first name) and my FIL Mr. Michael.
My DH calls my parents Mrs. Lisa and Mr. Tommy.
I find it really strange when people call their MIL by just their first name. I was raised to call someone older than me Mr. or Mrs. until they tell me not to use Mr. or Mrs.
Are you the same way, or more casual with using just their first name or a nickname?