In the first instance they are the believers of the New Testament era who were martyred for their faith (Revelation 6:9–10). The addition of the word “all” broadens the reference to all believers who suffer on account of the faith (as all would in some form, see John 15:18–20, John 16:2). This includes the saints of all ages, also those on earth today.
3 And another angel came and stood at the altar with a golden censer, and he was given much incense to offer with the prayers of all the saints on the golden altar before the throne,