The Torah, “Numbers,” 3:11 – 3:13: The Lord spoke to Moses, saying. As for Me I have taken the Levites from among the children of Israel in place of all first-borns among the children of Israel who have opened the womb, and the Levites shall be Mine. For all the firstborns are Mine; since the day I smote all the firstborns in the land of Egypt, I sanctified for Myself all the firstborns of Israel, both man and beast they shall become Mine, I am the Lord.
Every new level begins from the Sefira of Keter, which contains everything. When it spreads it builds the other Sefirot. The Sefira of Keter is born from the previous state, which is the firstborn.
Question: Why are the firstborn of Israel compared to the firstborn who died in Egypt?
Answer: Because the firstborn who died in Egypt are egotistic desires that mustn’t be kept alive and so they had to die. It is thanks to this that a new type of firstborn appears. This means that the desires that were focused only on receiving for themselves die in their yearning and become the next level of the firstborn of Israel, the Sefira of Keter.
From KabTV’s “Secrets of the Eternal Book” 12/31/14