en_tn_condensed/exo/12/29.md

at midnight

"in the middle of the night"

all the firstborn in the land of Egypt ... all the firstborn of cattle

Here, "firstborn" always refers to the oldest male offspring. See how you translated a similar phrase in Exodus 11:5.

who sat on his throne

This refers to Pharaoh.

the firstborn of the person in prison

"to the firstborn of people in prison." This refers to prisoners, in general, and not to a specific person in prison.