To He Who struck Egypt through its firstborn. When the firstborn of Egypt heard Moses warn about the tenth plague, they rebelled and struck their elders, who were preventing the Israelites from leaving Egypt
Psalms 136:10, Rashi; Metzudot
When Moses declared G-d's intention to kill every Egyptian firstborn, they reacted frantically. Concerning their own fathers, they implored that Egypt release the Israelites so as to spare the lives of the firstborn. Their fathers replied, "Even if all Egyptians were to die, the Israelites shall not leave." So the doomed men turned to Pharaoh and asked him to avert the impending disaster. But Pharaoh, too, dismissed them and sent his servants to "beat then until they are humpbacked," Left with no alternative, the firstborn took up arms against their fathers and countrymen, slaying 600,000 of their own. The Psalmist alludes to this miracle with the words, Who struck Egypt through its firstborn. He does not say, Who struck the firstborn of Egypt" but says, "Who struck Egypt through its firstborn" (Midrash Tehillim).
Emotions are the offspring of the intellect. The firstborn of Egypt is Egypt's most prized child, its ability to love. The passions of Egypt are invested in the ephemeral pleasures of materialism. As long as we are dominated by Egypt's firstborn, we cannot exercise our love for G-d. Thus, on special occasions, such as the festivals, G-d strikes Egypt through its firstborn, silencing our unhealthy passions so that His kindness, the light of His love, which is forever, can shine brightly within us, allowing our innate love for Him to glow (Rabbi Schneur Zalman of Liadi).
Tehillim with Commentary