KJB
And knew her not till she had brought forth her firstborn son: and he called his name JESUS.
ESV
but knew her not until she had given birth to a son. And he called his name Jesus.
And- Joins Phrases or any sentence
but- is a contradiction , and exception an Objection.
Firstborn- The Legacy of the Father, The Heir to the Throne.
A son - one of many, just another son heir to nothing.
Do you think the difference matters, I do.