While I am not dogmatic about it, I think the Martyrs in view are those killed during the Tribulation, based on the surrounding text. It is given at the end of the tribulation, with correlation to the beast:
Revelation 20
King James Version (KJV)
1 And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand.
2 And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years,
3 And cast him into the bottomless pit, and shut him up, and set a seal upon him, that he should deceive the nations no more, till the thousand years should be fulfilled: and after that he must be loosed a little season.
4 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.
5 But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.
Only in this Age do we see the witness of Jesus, which begin after the Holy Ghost in the Ministry of Comforter had come:
Acts 1:8
King James Version (KJV)
8 But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
It is possible that one might consider all saints that have been martyred throughout history, but I take the position that only Tribulation Martyrs are in view. And while the Pre-Tribulation Rapture is rejected my many, the view I take has all martyrs, as well as those who were not martyrs, resurrected in the Rapture of the Church prior to the beginning of the Tribulation.
Revelation simply does not have a resurrection which fits the terms of the teaching concerning the Rapture of the Church. It can't be the First Resurrection, because the physically living saints are not glorified at this time. That then denies the general resurrection of the Great White Throne, because no mention of the unjust is in view in the Rapture. We could consider the Mid=Tribulation view, and speculate the Church is Raptured when the Two Witnesses are, but again, only those two seem to be in view.
God bless.