This too relates to a/the symbolically written thousand years that began with the first advent of Christ. Verse 6 is after vss 1-7 spiritually depicting Old Covenant faithful saints after the Lamb of God (Christ) was slain for sin and resurrected to heaven. They are called "the firstfruits unto God and to the Lamb." Why? Because they remained faithful unto death.
Re 14:1 ¶ And I looked, and, lo, a Lamb stood on the mount Sion, and with him an hundred forty and four thousand, having his Father's name written in their foreheads.
Re 14:2 And I heard a voice from heaven, as the voice of many waters, and as the voice of a great thunder: and I heard the voice of harpers harping with their harps:
Re 14:3 And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth.
Re 14:4 These are they which were not defiled with women; for they are virgins. These are they which follow the Lamb whithersoever he goeth. These were redeemed from among men, being the firstfruits unto God and to the Lamb.
Re 14:5 And in their mouth was found no guile: for they are without fault before the throne of God.
After the 144,000 Old Covenant children of all the tribes of Israel, (Rev 7) were sealed by the Holy Spirit sent from Christ, they ascended with Christ to heaven after He resurrected from the dead.
Eph 4:8 Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men.
Eph 4:9 (Now that he ascended, what is it but that he also descended first into the lower parts of the earth?
Eph 4:10 He that descended is the same also that ascended up far above all heavens, that he might fill all things.)
Once the Old Covenant faithful saints have ascended to heaven a spiritual body as living souls even after death, the time for proclaiming the everlasting Gospel of the Kingdom of God through Christ had come to every nation, kindred, tongue and people of the earth. Before time for proclaiming the Gospel of the Kingdom of God unto all nations of the earth, Israel only had heard of the Gospel of the coming Messiah to redeem His people. Babylon fallen in this passage depicts the Covenant people of Old. This is not depicting the nation of Israel fallen, but the Holy City and Temple had fallen through spiritual apostasy and adultery. They were representative of the Congregation/Church of Old, they were appointed to be spiritual leaders but instead gave themselves over to serve evil under the influence of their father the Devil. Once Satan was bound through the death and resurrection of Christ, the New Covenant through Christ came to all the world with the Gospel. Babylon symbolizes the generation of evil, not an ethnic people, but an apostate people that would continue to grow causing all nations to drink of the same wine of fornication that she was guilty of.
Re 14:6 ¶ And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people,
Re 14:7 Saying with a loud voice, Fear God, and give glory to him; for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters.
Re 14:8 And there followed another angel, saying, Babylon is fallen, is fallen, that great city, because she made all nations drink of the wine of the wrath of her fornication.