**Where? When? This is an entirely inaccurate generalization. **
In the Bible, the firstborn were to be dedicated to the Lord God but not sacrificed.
First born animals had to be “redeemed”.
Years ago I read that during digs on old Phoenician ruins, often a sealed container with the words “CORBAN” (”Sacrifice” in Phoencian letters) scratched on it would be found. Inside were the burned bones of babies.
The law dedicates the first born because, when the Children of Israel made the blood atonement and splash the door frame with the blood of the “perfect lamb”, the angel of death passed over that household.
All the first born, which were not so covered, died that night.
This is the significance of the law’s singling out the first born.