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 the livestock

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 the dungeon

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