In Ezekiel 48, where he prophesied about a future temple, he talks about Kohanim and Levites serving. "Kohanim of the sons of Zadok..." or "Now for the Levites, opposite the border of the Kohanim..." and so on. However I have heard (don't remember where) that firstborn males are supposed to be the ones to serve in the Third Temple. So who will serve, the Kohanim and Levites, or the firstborn?
The Ohr Chaim on Bamidbar 3:45 writes that the firstborn will serve in the Third Temple. שאמרו ז"ל עתידה עבודה שתחזור לבכורות
Yonathan Eybeschutz writes in Ahavat Yonatan on the haftorah for Emor, that in the future, there will be atonement for the sin of the golden calf, and thus the firstborn will return to temple service.
I've seen many who are puzzled by the above Ohr Chaim, since it's not clear what Midrash/Gemara he gets his statement from. I did see an edition of the Ohr Chaim online that refers to the following Yalkut Shimoni 364:
"Everywhere the posuk say לי (=to me) it refers to something which remains forever and ever... by the firtborn the posuk states "כי לי כל בכור".
The implication being, that the selection of the firtborn remains in effect for eternity, and thus it must be that they will eventually return to serve in the temple.
I also found this essay on the subject: http://www.haoros.com/Archive/index.asp?kovetz=833&cat=1
It is mentioned in the Ohr Hachaim Bereishis 49:28 דבר... He mentions this idea that bechorim will serve in the future based on Chazal.There is more on this topic , this is what I have for now,ayin sham.