וַיְהִ֣י ׀ בַּחֲצִ֣י הַלַּ֗יְלָה וַֽיהוָה֮ הִכָּ֣ה כָל־בְּכוֹר֮ בְּאֶ֣רֶץ מִצְרַיִם֒ מִבְּכֹ֤ר פַּרְעֹה֙ הַיֹּשֵׁ֣ב עַל־כִּסְא֔וֹ עַ֚ד בְּכ֣וֹר הַשְּׁבִ֔י אֲשֶׁ֖ר בְּבֵ֣ית הַבּ֑וֹר וְכֹ֖ל בְּכ֥וֹר בְּהֵמָֽה׃
And it came to pass at midnight, that the LORD smote all the firstborn in the land of Egypt, from the first-born of Pharaoh that sat on his throne unto the first-born of the captive that was in the dungeon; and all the first-born of cattle.
Zohar
The verse, "flocks, and menservants, and women servants," refers to lower crowns of the Kelipot, which the Holy One, blessed be He, slew in Egypt. They are called "the firstborn of cattle" (Shemot 12:29), "the firstborn of the captive" (Ibid.), and "the firstborn of the maidservant" (Shemot 11:5). Esau took fright and came toward him; he feared Jacob as much as Jacob feared him.