There are many readings of this verse. Some believe that early Israelites and others did sacrifice their firstborn, and only later substituted the offering of animals.
Two issues come up for me. One is, what does it mean to make an offering to God. Are his/her emotions so fragile that he cares. Is he happier when an offering is made to him. Does he say to his wife, “Good day at the office, hon, the Israelites gave me lots of goodies today.”?
I like to take the position that Moses and/or the writers of the Torah knew that our connection to that which is beyond us was very important. We should not think of this or that being mine. What we earn is a product of what we are given. To give back helps us realize our interconnection with others. Nothing is so precious that we shouldn’t we willing to share it. So we do the offerings, which, after the destruction of the Second Temple, morphed into studying the Torah.
Second, what is this about giving your first son? This contradicts other places in the Torah where it says that the firstborn were originally the priests and later the Levites:
“Now behold, I Myself have taken the Levites from among the children of Israel instead of every firstborn who opens the womb among the children of Israel. Therefore the Levites shall be mine, because all the firstborn are Mine. On the day that I struck all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, both man and beast. They shall be Mine: I am the Lord.” (Numbers 3:12-13)
So now “They shall be Mine:....” has a different meaning. It means that they should devote themselves to my service. It is that they need to take care of their siblings and parents when they grow old. They are the leaders of the ever important family.
We (I’m a firstborn son) have a job to do.