In the Revelation you see that those who went through the events at the end of the age are seated on thrones. You see how in chapter 15, those who had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name, are seen standing on the sea of glass, having the harps of God (Revelation 15:2).
You see how in chapter 20, those who had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name, are seen seated on thrones, and they are alive [zao], and reigning with Christ for a thousand years (Revelation 20:4-6), where John is told that this is the first resurrection of the body [anastasis].
They are not said to have part in the firstfruits of the resurrection, because Christ alone is the firstfruits of the resurrection:
"But now is Christ risen from the dead, and become the firstfruits of them that slept. For since by man came death, by man came also the resurrection of the dead. For as in Adam all die, even so in Christ shall all be made alive. But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming." (1 Corinthians 15:20-23).
"And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years. (Revelation 20:4).
These are individual saints that John is talking about. No individual saint has ever lived a thousand years or reigned with Christ a thousand years.
"This is the first resurrection of the body [anastasis]. Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years." (Revelation 20:5b-6).
They are not said to have part in the firstfruits of the resurrection, because Christ alone is the firstfruits of the resurrection. It is through the power of His resurrection that the first resurrection of the body in the case of those who belong to Him at the time of His return, will occur.