Rev 5:9 is often cited as proof of Limited Atonement/Particular Redemption, the "L" of the Calvinist "TULIP." The New American Standard Version (1995 update) reads:
And they sang a new song, saying, "Worthy are You to take the book and to break its seals; for You were slain, and purchased for God with Your blood men from every tribe and tongue and people and nation."
The argument is that the glorified saints (v. 8) are praising God's objective redemption/atonement, through the sacrifice of Christ, for the elect who are among members of all ethnic groups.
Interestingly, there is a textual variation in the Greek texts. The NA29 reads:
καὶ ᾄδουσιν ᾠδὴν καινὴν λέγοντες· ἄξιος εἶ λαβεῖν τὸ βιβλίον καὶ ἀνοῖξαι τὰς σφραγῖδας αὐτοῦ, ὅτι ἐσφάγης καὶ ἠγόρασας τῷ θεῷ ἐν τῷ αἵματί σου ἐκ πάσης φυλῆς καὶ γλώσσης καὶ λαοῦ καὶ ἔθνους
The Byzantine text-type, however, reads the following (emphasis added):
Καὶ ᾄδουσιν ᾠδὴν καινήν, λέγοντες, Ἄξιος εἶ λαβεῖν τὸ βιβλίον, καὶ ἀνοῖξαι τὰς σφραγῖδας αὐτοῦ· ὅτι ἐσφάγης, καὶ ἠγόρασας τῷ θεῷ ἡμᾶς ἐν τῷ αἵματί σου ἐκ πάσης φυλῆς καὶ γλώσσης καὶ λαοῦ καὶ ἔθνους
A recent article in the Zeitschrift für die neutestamentliche Wissenschaft argues that the Byzantine text type reflects the original reading of the verse.
If his conclusions are correct (and the article is pretty solid), it means that the elders are speaking of themselves, not all those who were "particularly redeemed." To quote Malik's summary:
On inclusion of ἡμᾶς, the ἐκ-construction ceases to be the direct object, in which case the twenty-four elders must be taken as heavenly representatives of those who were redeemed “from every tribe, tongue, people, and nation.” This function of “heavenly representation” is in keeping with Rev 5,8 where the elders are said to worship God with harps and incense, the latter representing the prayers of the saints. In v. 10, the focus is then shifted from the heavenly representatives of the redeemed to the redeemed themselves: the Lamb “made them a kingdom and priests, and they shall reign over the earth.” It is the promise of the saints’ future reign that is most likely to have occasioned this shift to the third person—a universal promise yet to be fulfilled in those who follow the slain Lamb to the very end.