Musar על ויקרא 2:12

In earlier eras all such sacrifices were performed with firstling animals, as indicated by the phrase קרבן ראשית in Leviticus 2,12. Moses' whole mission to Pharaoh was to inform the latter that Israel's was G–d's true firstling, i.e. בני בכורי ישראל (Exodus 4,22). As a result it was appropriate that the Jewish people first and foremost should perform עבודה for G–d. The only reason Jacob had purchased the birthright from Esau was to qualify as the first-born in order to perform this service to G–d. Jacob would not otherwise have been been qualified to perform this service, [at a time when the Levites had not yet replaced the first-born as priests. Ed.]