There is a tension in every family between the older child and all the subsequent children, an assumption that being “the first born” comes with an inherent status. The idea in English Law (and in many ancient legal codes as well, including parts of Ancient Egyptian history) of primogeniture is tied to this: the first-born is special, and deserves preferential treatment for no other reason than birth order.
The Torah handles the concept quite differently. Cain is born first, but G-d favors Abel’s offering instead. The lesson is simple enough: G-d values what we do, not merely how we are created.
The pattern repeats numerous times. Ishmael is Avraham’s first-born – and Isaac is chosen instead. Esau is older than Jacob, but the younger Jacob continues the Avrahamic line. And Jacob’s first three sons, Reuven, Shimon and Levi are all disqualified (as a result of their choices) from leading the people: Yehudah is selected instead. Even Efrayim and Menasseh are blessed by the reverse of their birth order. The lesson, at least to the Jewish people themselves, could hardly be clearer: our actions, not our birth order, are what define us and give us value in the eyes of each other and in the eyes of G-d.
This lesson, however, was a local one. The rest of the world continued to hold to the notion that the first-born is always superior, that the older is on top of every hierarchy.
I suspect that part of the reason for this is tied to biological creativity itself: the first-born is the proof of the potency and vitality of the father, the visible trophy of manliness. In a pagan world that recognizes deities as incarnating forces (the sun deity, river deity, etc.), creative power was a force unto itself, and just as older deities are generally considered more powerful than younger ones, so, too, the first-born is assumed to have inherited more of the father’s vitality.
There is something to this: creative power is indeed important to our existence as people. Which is why the Torah commands us to offer the first-born to G-d – not because the first-born is necessarily superior in any measurable sense, but because by acknowledging that our creativity is on loan from G-d, shows that instead of claiming the credit for ourselves, we choose to explicitly thank G-d for the creative powers with which we are endowed.
People are, of course, not the only first-borns. In the ancient world, the oldest “real” nation (and the first organized kingdom from Ham, Noach’s son), was Egypt. The Kingdom was the center of its universe: thanks to the Nile River and the Mediterranean, Egypt had an enormous advantage in terms of food production and trade.
The Ten Plagues, as has been argued quite persuasively by others here and here as but two of many examples, were to show G-d’s superiority over each of the Egyptian deities in turn: the Nile, Heqat the Frog god, etc. But the last plague is much harder to understand – if, as is usually argued, it is a strike against Pharaoh, then why did all the first-borns (man and beast alike) in the land have to die?
I think that the enhanced understanding of the role of the first-born in Judaism explains this well: by striking at the first-born throughout the land, G-d is not singling out any deity – He is, instead, attacking the notion of the superiority of Egypt herself based on her status as the first-born nation in the ancient world.
G-d is telling the Egyptians (and the world) what the Jews already knew: the first-born may have pole position, but it is our choices and actions, not our birth order, that determine the extent of our relationship with G-d. We could expand this even more: every single person is born into a circumstance that is unique in some way. By killing the first-born of Egypt, G-d is telling the world that people are not judged according to where they begin life, but by what they choose to do during their lives.
All of this is connected to the imagery of the Exodus from Egypt being a national birth – coming through the waters, being born as a nation. Seen in this way, the plagues could be compared to birthing pains afflicting the mother/Egypt, while the unborn baby is basically carried along for the ride without any undue stress.
When the Jews leave Egypt, then, they are the newborn, clearly not G-d’s firstborn child, since we came out of a host nation that was much older than we were. But Egypt did not credit G-d for any of the blessings and riches that they enjoyed: they believed that it was their entitlement, part-and-parcel of being the first-born.
So when G-d takes away their first-born, there is an aspect of middo kneged middo, measure-for-measure. Because the Egyptians did not credit G-d for their creative powers, G-d took away their first-fruits, their most creative acts.
One might even go so far as to suggest that the Torah is telling us that dynamism is more important than stasis: a philosophy that celebrates choices over status is one that inherently seeks change and growth, the kinds of traits that separate Judaism from fate-based philosophies. This is a worldview that seeks a dramatic arc to each person’s life, as well as to the lives of nations: history as progress, as a journey to somewhere, instead of the more common ancient view of a cyclical, endlessly repeating set of patterns.
This even ties into the status of the Jewish people ourselves. When we were in Egypt, we were Pharaoh’s slaves. When G-d seeks to free us, He did not tell Pharaoh that we belonged to a different master. Instead, the words used are verbs: Pharaoh should free the Jewish people so that we could serve G-d, not be servants to G-d. It is always the action that matters, not the status.