when Terah was 70, he begat 3 sons.

[Enoch987]

Genesis 11-12
Nahor age 29 begat Terah.
Nahor lived 119 years then died.
Terah age 70 begat Abram, Nahor, Haran.
Nahor Jr. married Haran's daughter.
Both were approximately the same age because Haran is the oldest brother.
Haran dies.
Terah and Abram moved to the city of Haran.
Terah dies at age 205.
Abram buried Terah. Abram was age 75.
130 years separate their ages.

Likely Nahor Jr. was named for Nahor who may have died the year earlier.
If so, Terah was 120 when he begat Nahor Jr.
It follows that Abram was born 10 years later. What we know for sure is Terah was 130 when Abram was born.

Abram is the firstborn because he is listed first.
Genesis 6-10. When Noah was 500 years, he begat Shem, Ham, Japheth.
In Noah's 600th year, the Flood came.
2 years after the Flood, Shem at age 100 begats his 3rd of 5 sons named Arpaxad.
Japheth is the oldest. The Catholic Bible helps God with His math and calls Japheth the youngest. Genesis 10:21.

How does this inform our understanding as Jesus as the firstborn son of God. Colossians 1;15 and Hebrews 12;23.
God said of Jesus, 'This is My chosen Son.' Luke 9:35.
This is not in the King James Bible and is not in Matthew and Mark account of the mount of Transfiguration.
The King James and Matthew/Mark have "this is My beloved Son."
The modern Bibles' (including the Catholic bible) translation of Luke 9:35 is from the earliest known manuscripts of the Bible.
My best guess is since their is only one Son of God according to the Creeds, the math of the firstborn does not apply to Jesus.

The 120 year prophecy. My Spirit will not always strive with man. His years will be 120 years. Genesis 6:3.
When Japheth was 120, Shem was 117 and Arpaxad was 17.
When Terah was 120, Nahor Jr. may have been born.
When Terah was 130, Abram was born.
When Jacob was 130, he reunited with Joseph after he thought his favorite son had died 22 years earlier.
This was 2300 years after Adam was created and it divided the 430 years that Israel would be a foreigner in a foreign land in half (215 years from Abram receiving God's promise to Jacob's reunion plus 215 years to the Exodus.
When Adam was 130, he begat Seth. Seth was Adam's 3rd son. He carried on the righteous line of Christ.

 

[Enoch987]

Jacob was 120 when his father Isaac died at 180. Oddly, Isaac is written out of the story after Rachael's death. Isaac was legally blind since before Jacob left to live with Laban.
Jacob thought Joseph died at age 17. He became second to Pharoah at age 30. about the same age that JEsus started his ministry and the age that King David became king of Israel.
After 7 years of plenty and 2 years famine, Jacob reunited with Joseph. 5 years of famine remained.
Joseph was 39 years at the reunion compared to Jacob who was 130. Jacob left Isaac in his early to mid 70's then spent 20 years working for Laban.
Jacob is the 22nd generation.
Joseph is the 23rd generation.
It was 2300 years since Adam's creation.
the number 23 represents Jesus in the bible. First Corinthians 10:8
Jacob lived for 17 years after the reunion.
Jesus resurrected on the 17th day of the Jewish year.
If JEsus' birthday was 2/7th of a year after his death on the 14th day of the year, he would have been born on the 117th day. around July 18, it depends on when the Jewish year starts.
There are 117 days from Pentecost (the 14th day is Passover plus 50 days = 64)
the 64th day plus 117 = 181.
the 181st day is the first day of the 7th month , the day of repentance for the Jews.
I think JEsus was born in the summer months.