I am going to partly summarise the very useful answers and strive to bring out that which all seem to have headed towards but never fully arrived at. (no criticism intended)
The accomplishment of Jesus' mission is central to everything since Gen 1:1 (or whatever came before that)
Jesus is the newborn King and holy child as noted in the Gospels, but IF he had failed in his mission - he would have stayed dead and so would all of us.
However, God's (inspired biblical) focus is firmly on what Jesus did by remaining sinless unto death and succeeding where the 1st Adam failed - expectedly. It is Jesus' NEW LIFE - the REbirth that God wants us to set our eyes, hearts and thoughts on - indeed, our very lives!
That new life at his resurrection and subsequent ascension is the important bit. The Eucharist/Lord's Supper/communion is God's and Jesus' focus and want it to be ours too! Jesus has, as 'firstborn from the dead' included all men in that new life. (all are included - not all are privy to knowing just yet)
What does dead mean?
Dead is not absence of physical life, but the state of being permanently DEAD! Meaning, unable to be raised to a new life ever. We have been born into a corrupted world and sinfulness is our state - earning ALL the eternal death penalty. Clearly, there is a plan in place to mitigate this unfortunate situation.
All are dead to sin, except Christ who beat evil.
Jesus is not just raised from the dead - as Lazarus was, He is raised from a physical death with spiritual consequences for all! He defeated death, it's power, and it's sting of sin - by remaining sinless, he has qualified to have eternal life through rebirth. Because of God's grace, all men are freed from death in HIM and ONLY in him, they too may share this same eternal life!
1 Pet 3: 18 ...For Christ also died for sins once for all...so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit
Romans 6:9 knowing that Christ, having been raised from the dead, is
never to die again; death no longer is master over Him.
Hebrews 2:14 Since the children have flesh and blood, he too shared in
their humanity so that by his death he might break the power of him
who holds the power of death--that is, the devil.
The cause of death - sin, is defeated in Jesus' one sacrifice for all for ever - only fully realised at our rebirth - at resurrection or transformation, at Jesus' return. Rom 8:3 '...in the likeness of sinful flesh and as an offering for sin, He condemned sin in the flesh'.
Only in Jesus are we 'made in the image of God'. As he was also (2 Cor 4:4)
1 Cor 15:49 Just as we have borne the image of the earthy, we will
also bear the image of the heavenly. (See Note#1 at end)
1 Cor 15:51 the dead will be raised imperishable, and we will be changed.
The 'dead' are not just those in the grave. All men are dead to sin and would remain so unless we accept Jesus' death for us.
Eph 2:1 And you were dead in your trespasses and sins...
Because Jesus 'died for all' 2 Cor 5:15, all are made fully alive (at different times) in the spirit -- just as it was for him, so it will be for us. We, who were irrecoverably spiritually dead, are able to join in with him, the first-fruit of the first-fruits - all believers are the 'many other' brethren raised to eternal life Rom 8:29.
This includes, and will eventually impact, all who ever lived since humanity began - as all are dead to sin.
What does 'firstborn' signify?
His rebirth is a precursor to ours - dead to the flesh, made alive (reborn) in spirit - at his resurrection. He is the 'firstborn - of many brethren' who also are dead to the flesh and in Him made alive in the spirit - which death can never approach - as it cannot Jesus anymore.
Rom 6:9 Christ, having been raised from the dead, is never to die again.
Jesus IS the firstborn from the dead. He is the first and only thus far to have been granted passage from physical existence to spiritual life.
He is the 'firstborn of many brethren' (Rom 8:29) - the first of many who will follow him to join the Father. This ties in with his 'rebirth' or being born again of the spirit, as all believers will be when resurrected. (John 3:6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit)
Jesus, as 'firstborn from the dead', 'foreknown' and 'slain' from the foundation of the world - born and died in the flesh - raised in the spirit. In HIM all creation will be redeemed - all according to the plan, the design, the word of God - made flesh. To live and to die as the Lamb of God and be raised eternal so that all men might truly live - in him, forever!
As Nigel has shown below there is much more to this 'firstborn' or first-fruits symbolism. There was a special festival observed in all Israel. Originating in OT times it was a type of he who was to come. I include very brief excerpts from a site I'll link to later.
The primary instruction regarding the wavesheaf is found in Leviticus 23:9-14:
And the Lord spoke to Moses, saying, "Speak to the children of Israel, and say to them: ‘When you come into the land which I give to you, and reap its harvest, then you shall bring a sheaf of the first-fruits of your harvest to the priest. He shall wave the sheaf before the Lord, to be accepted on your behalf; on the day after the Sabbath (Sunday morning)the priest shall wave it. And you shall offer on that day, when you wave the sheaf, a male lamb of the first year, without blemish, as a burnt offering to the Lord. In its setting in the Old Testament, the wavesheaf offering represents a thankful acknowledgment to God as the Giver of the harvest, while dedicating or consecrating it to Him. Its waving set the stage for the rest of the harvest to proceed. In fact, the work of harvesting could not begin until the wavesheaf offering occurred.
Everything God does is important - even if we don't or didn't understand what His command pointed to - this is one strategic example that has turned a basic physical routine into a key spiritual marker of the first-fruit (Jesus) of all first-fruits - the 1st resurrection!
(Note; I do not subscribe to all the particular beliefs mentioned on this page, but the items quoted are quite useful and valid)
Note#1 Rom 8:29 'He also predestined to become conformed to the image of His Son, so that He would be the firstborn among many brethren'
'Made in the image of God' was a process begun with Adam #1, which was interrupted by sin. It is restored through Jesus - Adam #2 - as it was intended from before the beginning. We are only in the image of God, in Jesus.
Jesus IS the image of God - if we are conformed to be in Jesus' image, we will be in God's image in the way that God has planned.