The role of firstborns is very prominent in Parshas Bo. Hashem smote the firstborn sons and animals of Egypt, while sparing those of B'nei Yisroel. In commemoration, Hashem commanded that firstborns be sanctified and redeemed, as presented at the end of the parshah.

Why is the firstborn so important in the story of Yetzi'as Mitzrayim? Rav Yosef Dov Ha-Levi Soloveichik explained that the firstborn Egyptians were primarily responsible for the enslavement of the Jews in Mitzrayim, as the firstborns in ancient cultures bore the positions of leadership. (See Hagaddas Si'ach Ha-Grid, s. 47.)

Perhaps an alternative explanation can be offered. We find that Hashem initially commands Moshe, "And you shall say unto Pharaoh, 'Thus declares God: Israel is my firstborn.'" (Shemos 4:22) Rashi thereupon quotes Medrash Shemos Rabbah: "At this point, Ha-Kadosh Baruch Hu signed off on the sale of the birthright to Yaakov from Eisav."

This Medrash indicates that the b'chorah (birthright) that Yaakov purchased from Eisav, which gave Yaakov a leadership role in Avodah, Hashem's service, was being bestowed upon B'nei Yisroel on a national level, such that their redemption from Mitzrayim was to bring them near to Hashem and designate the Jewish nation as leaders in Avodas Hashem.

The downfall of Egypt's firstborns and the salvation of the firstborns of B'nei Yisroel was thus in fulfillment of Hashem's message to Moshe that B'nei Yisroel would occupy the position of leadership in Avodah; the substitution of Jewish firstborns for Egyptian firstborns comprised Hashem's formal repudiation of universal leadership in Avodah and the particularization of leadership in Avodah with B'nei Yisroel. This explains why the firstborns of B'nei Yisroel had to be treated with sanctity, and mere thanksgiving for being spared did not suffice, for their fate was not merely one of individuals who were spared from suffering. Rather, the fate of the Jewish firstborns was one of elevation, for the role of Divine service, as represented through them, was transferred to B'nei Yisroel on Pesach night.