וּמֵ֣ת כָּל־בְּכוֹר֮ בְּאֶ֣רֶץ מִצְרַיִם֒ מִבְּכ֤וֹר פַּרְעֹה֙ הַיֹּשֵׁ֣ב עַל־כִּסְא֔וֹ עַ֚ד בְּכ֣וֹר הַשִּׁפְחָ֔ה אֲשֶׁ֖ר אַחַ֣ר הָרֵחָ֑יִם וְכֹ֖ל בְּכ֥וֹר בְּהֵמָֽה׃
애굽 가운데 처음 난 것은 위에 앉은 바로의 장자로부터 맷돌 뒤에 있는 여종의 장자까지와 모든 생축의 처음 난 것이 죽을지라
Rashi
עד בכור השבי UNTO THE FIRSTBORN OF THE CAPTIVE (these words are used in 12:29 instead of עד בכור השפחה “unto the firstborn of the handmaid” in this verse) — Why were the captives smitten, for they had not enslaved the Israelites? In order that these might not say that their god had claimed satisfaction for the humiliation imposed upon them and had brought this punishment upon the Egyptians (cf. Mekhilta).
Rashbam
אשר אחר הרחיים, later on the simile is changed and the Torah speaks about השבי אשר בבית הבור, “the prisoner in the dungeon.”(12,29) There is no contradiction, seeing that we know from Judges 16,21) that Shimshon as well as other prisoners were obliged to act as the “horse-power” turning the millstones while they were incarcerated.
Sforno
מבכור פרעה עד בכור השפחה, from the highest layer of Egyptian society to the most lowly. When we read a similar line in 12,19 the meaning is “from the most severe sinner to the relatively least guilty sinner.”