# midnight

This is the time of 12 am (at night) or 2400 hours.

# All the firstborn ... the firstborn of Pharaoh ... the firstborn of the slave girl ... the firstborn of the cattle

The "firstborn" always refers to the oldest male offspring.

# who sits on his throne

This phrase refers to Pharaoh.

# who is behind the handmill grinding it

"who is grinding at the handmill" or "who is behind the handmill grinding grain"