Some have been confused by the story of the raising of Lazarus (Jn 11). Obviously, Lazarus was brought back to life prior to the resurrection of Christ. However, Lazarus was not resurrected according to Alma's definition, for he was brought back to a mortal state. His mortal body did not put on immortality, nor did his corruption put on incorruption.
Hence the term "firstfruits" does not mean that Christ was the first dead person to be brought back to life. Rather, it means that he was the first to be resurrected with an immortal body.