I don't think you need mods to inherit traits. I didn't have mods in my game when Heather had her son Perry. The father was an imaginary friend. I noticed something odd with him as a child and I had put mods into my game by then (about 2 years later) and he had the hidden trait of pyromaniac.
Other children born to imaginary friend fathers in my games have had the imaginary friend hidden trait.
There is a list somewhere that shows what hidden traits various sims can inherit and it isn't to do with mods. But mods might influence it too if you have mods in the game.
I think the firefighter's offspring will inherit pyromaniac. There is another that lets them inherit the burglar hidden trait. I've had one of my sims years ago who I think must have had it. It was years before I had mods in the game and she could sneak into houses. It was very useful when in World Adventures and your sim got an adventure that required them to do a tomb that was in the home of a sim who was assigned to be a merchant so was never home.
There is a child (or toddler) from Barnacle Bay who I had in my boarding school. He is a pre-made sim and I checked his traits one day and saw he has the hidden burglar trait too.
This link will give you a list of hidden traits and what careers pass the hidden traits on to their offspring.
https://www.carls-sims-3-guide.com/traits/
It seems that Natural Sneak is inherited from the Criminal Career.