The last and final plague, the death of the firstborn, brought Pharaoh to submission, forcing him to let the Israelites go. What is the significance of the firstborn? Exodus says,

“Sanctify to me all of the firstborn, whatever opens the womb among the children of Israel, both of man and of animal. It is mine.”
Exodus 13:2 explains the importance of the בְּכוֹר (bechor), or firstborn,

“Our firstborns achieved special status when, although our nation was spiritually fallen in Egypt and quite similar to our Egyptian neighbors, G‑d spared us during the Plague of the Firstborn. . . The Chinuch adds that this reminds us that everything in the world belongs to G‑d. When we consecrate our very first and very best, we are reminded that everything really belongs to our Creator, and that we must “purchase” it from Him before using it. The Maharal (Rabbi Judah Loew of Prague, 1525-1609) explains that since G‑d is the first being, it is fitting that firstborns are consecrated to Him.”, Pidyon HaBen, What & Why [2]

R’ Ari Kahn comments on the differences between Cain and Abel,

“…being a firstborn of G-d is about how we lead our lives, it is the manifestation of the image of G-d within, not a question of sequence of birth.”
R’ Ari Kahn, M’oray HaAish, Bo: The Firstborn, [3]

Exodus Rabbah links the position of the Firstborn to Israel, and the King Messiah,

”SANCTIFY UNTO ME ALL THE FIRSTBORN (Ex. 13:1). R. Nathan said: ‘The Holy One, blessed be He, told Moses: ‘Just as I have made Jacob a firstborn, for it says: ‘Israel is My son, My firstborn (Ex 4:22) so will I make the King Messiah a firstborn, as it says: ‘I also will appoint him firstborn’ (Ps. 89:28).”
Exodus Rabbah 14:7, Soncino Press Edition

Pesikta de-Rab Kahana comments,

“R. Berechiah taught in the name of R. Abba bar Kahana: Through the merit of obeying the precept ‘Take ye on the first day, etc., I [says God], shall reveal Myself to you as “the First,” and in your behalf influction punishment upon “the first,” and build you “the first” and bring you “the first”. I shall reveal Myself to you as “the First” for it is written of Me, ‘I the Lord am the first, I the last – I am He’ (Isa. 41.4), and will inflict punishment in your behalf upon “the first” – upon the wicked Esau, of whom it is written, ‘The first came forth ruddy (Gen. 25:25), and build you “the first” – the Temple, of which it is written ‘Throne of glory on high from the first, counterpart of the place of the Sanctuary’ (Jer. 17:12); and bring you “the first” – the Messiah, of whom it is written, ‘The first unto Zion will I give, [who will say] “Behold, behold them [returning to Zion], and to Jerusalem a messenger of good tidings.’ (Isa. 41:27)”
Pesikta de-Rab Kahana, Piska 27.10, translated by William G. Braude, Jewish Publication Society, pg. 566

The Midrash and Zohar explain (based on Proverbs 8:22-30) that one of the names of the Torah is ‘Reisheet, the “First.” Paul identifies the Torah, the Instrument of Creation, to the Messiah,

“The Father (has) made us fit to be partakers of the inheritance of the holy ones in light, who delivered us out of the power of darkness, and translated us into the Kingdom of the Son of his love, in whom we have our redemption, the forgiveness of our sins, who is the image of the invisible G-d, the Firstborn of all creation. For through him all things were created, in the heavens and on the earth, things visible and things invisible, whether thrones or dominions or principalities or powers, all things have been created through him, and for him. He is before all things, and in him all things are held together. He is the head of the body, the assembly, who is the Beginning, the Firstborn from the dead, that in all things he might have the preeminence. For all the fullness was pleased to dwell in him, and through him to reconcile all things to himself, by him, whether things on the earth, or things in the heavens, having made shalom through the blood of his cross.”
Colossians 1:10-20

haggadah Section: Conclusion