Parashas Bo · Shemos 12:12 · TT note 2
Torah Temimah on Shemos 12:12, note 2 (on וְהִכֵּיתִי כָל בְּכוֹר) links to Beha'aloscha (“כִּי לִי כָּל בְּכוֹר”) and classical sources on bechorim before the Mishkan.
Sefaria — Exodus 12:12
Sefaria — Torah Temimah on Exodus 12:12
Sefaria — Numbers 8:17
English: “For all the firstborn are Mine … on the day I struck every firstborn in the land of Egypt” — from here [we learn] that before the Mishkan was established, the service was performed by the firstborn.
Linkage of the day of the plague with the consecration of Israel’s firstborn for sacred service.
On the day Egypt’s firstborn were struck, Israel’s firstborn were set apart for service, until the Leviim later took their place.