Answer:
I have honestly never studied this in great detail so I must admit I could not immediately answer this. Good question.
I remember always thinking it was the male child only but I also remember the 10 Commandments movie with Charleton Heston where there was a girl who they tried to put blood on her doorpost so she would not die since she was also the firstborn of her family. Of course, there were many inaccuracies in the movie but it made me wonder.
I went back to the original Hebrew and the word used for firstborn was pronounced “bek-ore” which was used for firstborn and also the “chief or eldest son”. So according to the original Hebrew word it was the firstborn male.
Interesting also that Pharaoh had commanded the male sons of the Hebrews to be killed and it seems as though the same punishment he inflicted on Israel came back upon him and his household!!!
Hope that helps,
Pastor Brad