מדרש אויף תהילים 89:28

“Take a census of…,” that is what is written: “He will not withdraw His eyes from the righteous…” (Job 36:7). The Holy One blessed be He does not withhold from them the realization of their ideal, as we found that Jacob coveted the birthright for the sake of Heaven, so he would be able to sacrifice. He purchased it from Esau with money and the Holy One blessed be He agreed with him, and He called him: “My son, My firstborn” (Exodus 4:22). He granted greatness to the firstborn so they would sacrifice before Him. That is, “He will not withdraw His eyes [einav] from the righteous” (Job 36:7). [Another explanation:] “Einav” is nothing other than in his place [taḥtav], just as it says: “And, behold, the mark maintained its appearance [be’einav]” (Leviticus 13:5). What is taḥtav? These are his children, just as it says: “In the wake of [taḥat] your fathers, your sons will be” (Psalms 45:17).
“And kings upon the throne” (Job 36:7), as the Holy One blessed be He accorded honor to the firstborn, and it was fitting for them to take the kingdom, as it is stated: “But the kingdom he gave to Yehoram, because he was the firstborn” (II Chronicles 21:3). Likewise, regarding David it says: “As for Me, I will make him My firstborn, supreme over kings of the earth” (Psalms 89:28). That is, “and kings upon the throne.”
“He has seated them forever” (Job 36:7), as they [the Israelite firstborns] would have been fit to take the priesthood and what the Levites perform, had they not sinned in the act of the calf. Initially, the firstborn would sacrifice, as it is stated: “He sent the young men of the children of Israel and they offered up burnt offerings [and they slaughtered feast-offerings of bulls to the Lord]” (Exodus 24:5).
Likewise, Jacob said to Reuben: “Reuben, you are my firstborn…[greater in honor [se’et] and greater in power [az]]” (Genesis 49:3). Se’et, this is priesthood, just as it says: “Aaron raised his hands [vayisa] toward the people, and blessed them” (Leviticus 9:22). Az, this is kingdom, as it is stated: “He will give strength [oz] to His king” (I Samuel 2:10). Had Reuben not sinned in the act of Bilha, he would have been worthy to take priesthood and kingdom, because he was the firstborn..
From where is it derived that they were worthy for the service of the Levites? It is as you find that the common Levites entered in place of the firstborn of Israel, as it is stated: “You shall take the Levites for Me, I am the Lord, in place of all the firstborn among the children of Israel” (Numbers 3:41). That is, “he has seated them forever [lanetzach].” This is the status of the Levites that the firstborn were worthy to take, just as it says: “The Levites were counted from thirty years old and above…of these…” (I Chronicles 23:3–4). “To supervise [lenatzeach] the labor of the House of the Lord” (Ezra 3:8). Regarding the tribe of Levi, the verse said: “My eyes are on the faithful of the land; they will dwell [lashevet] with me” (Psalms 101:6). That is “He has seated them [vayoshivem] forever.”
“They were exalted” (Job 36:7). Who caused the firstborn to lose this honor? It is because they exalted themselves and worshipped before the calf. Therefore, “if there are those bound in shackles [bezikim]” (Job 36:8), they were prohibited from sacrificing an item that is done in fire.⁵"An item that is done in fire" refers to a sacrifice. Zikim is nothing other than fire, as it is stated: “Behold, all of you are igniters of fire, [lighters of sparks] [zikot]” (Isaiah 50:11). [“Or trapped in the pangs of poverty” (Job 36:8)], that each and every one of them is obligated to redeem himself with five silver shekels and give them to the Levites.
We find regarding the firstborn of Israel that they descended from their prominence because of the act of the calf. But the firstborn of the Levites, who did not err regarding the calf, the priesthood was given to Aaron, who was the firstborn, and the rest of the firstborn Levites required neither redemption nor atonement, and merited Levite status with their brethren.
Were a person to whisper to you: ‘Was Gershon not firstborn? Why did the verse give precedence to Kehat in taking a census, and only then counted Gershon?’ You, too, say to him: ‘Because [the family of] Kehat were the bearers of the Ark, which is of the most sacred order, and Aaron the priest, who was of the most sacred order, emerged from him, while Gershon was sacred. That is why the verse gave Kehat precedence.
From where do you derive that Gershon did not lose his prominence because he was firstborn? It is as you find just as it is stated regarding Kehat: “Take a census of the sons of Kehat” (Numbers 4:2), so, it is stated regarding Gershon: “Take a census of the sons of Gershon [as well]” (Numbers 4:22). What is it that the verse stated: “As well”? It is so you will not say that it is for this that he counted the sons of Gershon second, because they are inferior to the sons of Kehat. No, rather, “as well” is written; that the sons of Gershon, too, were the equivalent of the sons of Kehat, but the verse gave them precedence here in deference to the Torah. But in other places, it gave precedence to Gershon over Kehat.

(Ibid., cont.:) BECAUSE THERE WAS A GREAT CRY…. How so? When an Egyptian marries five women, he has five first-born from them; and likewise when a woman is married to five men, she has from them five first-born {from them}. So all of them died. It was in order to confirm what is stated (in Exod. 11:5): AND EVERY FIRST-BORN <IN THE LAND OF EGYPT> SHALL DIE. Moreover, a house in which there was no first-born had an outcry {greater than the house} [as great as in the house] in which there was a first-born, because the master of the house was <himself> a first-born,⁷⁶I.e., by virtue of his status the head of the house was considered a first-born. as stated (in Ps. 89:28 [27]): I WILL ALSO APPOINT HIM FIRST-BORN. Thus it is stated (in Exod. 12:30): BECAUSE THERE WAS A GREAT CRY IN EGYPT; FOR THERE WAS NO HOUSE WHERE THERE WAS NOT SOMEONE DEAD.

“Consecrate to Me every firstborn; the first issue of any womb among the children of Israel, whether man or animal, is Mine” (Exodus 13:2).
“Consecrate to Me every firstborn” – Rabbi Natan says: The Holy One blessed be He said to Moses: ‘Just as I made Jacob the firstborn, as it is stated: “Israel is My firstborn son” (Exodus 4:22), so I will make the messianic king the firstborn, as it is stated: “I also will appoint him firstborn” (Psalms 89:28). So, “consecrate to Me every firstborn.”¹²The midrash is explaining that the superfluous word “every” teaches that there is another firstborn who will be consecrated in the future – the messiah. Caution Israel, just as I created the world and I said to Israel to remember the Sabbath day to commemorate the act of Creation, as it is stated: “Remember the Sabbath day,” (Exodus 20:8), so you should remember the miracles that I performed on your behalf in Egypt, and remember the day that you departed from there, as it is stated: “Remember this day that you departed from Egypt” (Exodus 13:3). Why? “Because with strength of hand the Lord took you from Egypt” (ibid.).¹³The midrash has “took you from Egypt.” The actual verse is “took you from this.”
“No leaven shall be seen with you [in all your borders] for seven days” (Deuteronomy 16:4), corresponding to the seven days between the redemption and the splitting of the Red Sea. Just as initially there were seven days of Creation and just as Shabbat takes place once every seven days, so shall these seven days be observed each and every year, as it is stated: “You shall observe this ordinance at its appointed time from year to year” (Exodus 13:10). It is analogous to a king who married a woman who was overseas.¹⁴The marriage took place remotely, while she was still abroad. Waves reached her [i.e. she encountered a storm] before she came to him. He said to her: ‘Do not remember all the waves that passed over you, but rather, on that day that you were spared from them, remember me and make a celebration every year.’ So with regard to Israel, the Holy One blessed be He appeared to them to redeem them, and some harsh waves passed over them, but He provided salvation for them. That is why He cautioned them that they should celebrate every year, as it is stated: “Rejoice in the Lord, and be glad, righteous ones” (Psalms 32:11).