Then what does Scripture mean when it says that Jesus is the firstborn of the dead?
Jesus informed us at John 3:13 that No one has ascended to heaven ...... that included King David - Acts 2:34
So, the first person resurrected to heaven- firstborn from the dead - would be Jesus.
In other words, No one would go to heaven before God would resurrect Jesus back to his previous spirit body - Colossians 2:12 B.
Unlike Jesus, Adam had No previous spirit body but only a physical body.
Adam was only offered everlasting life on earth as long as Adam kept God's Law.
Adam went from non-life, to life, and returned to non-life - Genesis 3:19
A person can Not ' return ' to a place he never was before. Adam returned to the dust.
In a nut shell, Jesus opened the way to heaven for some - Revelation 20:6; 5 vs 9,10
They have first or earlier resurrection than the majority of mankind.
The majority of mankind will have a future resurrection on earth during Jesus' coming 1000-year reign over earth.
That is why Acts 24:15 uses the ' future tense ' that there ' is to be ' a resurrection....'
The majority of mankind will have the same original opportunity that was offered to father Adam before his downfall.
Jesus will undo all the damage Satan and Adam brought upon mankind. - Rev. 22:2
We are nearing the coming ' time of separation ' of Matthew 25 vs 31,32 when the humble ' sheep' can remain alive on earth, and continue living on earth right into the start of Jesus' millennium-long day of governing over earth.