Who is the firstfruits and who is the next group resurrected after Him?
Christ is the firstfruits, the first company in resurrection. The next group resurrected after Him is the church age believer—those who are in Christ, part of the bride and body of Christ—who will be raised at His coming, according to their own order or company.
Les explains that resurrection is not general but ordered, like a military organization with platoons, companies, and battalions. Christ is the firstfruits, one company. The next company is the church age believer: those in Christ, part of the bride and body of Christ, resurrected at His coming. The context also notes that believer and unbeliever are both resurrected by God's power, but to totally different ends.
But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming.