About Interpreting the Bible
"And all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts." (Exodus 11:5)
"For I will pass through the land of Egypt this night, and will smite all the firstborn in the land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the LORD." (Exodus 12:12)
"And it came to pass, that at midnight the LORD smote all the firstborn in the land of Egypt, from the firstborn of Pharaoh that sat on his throne unto the firstborn of the captive that was in the dungeon; and all the firstborn of cattle." (Exodus 12:29)
"And Pharaoh rose up in the night, he, and all his servants, and all the Egyptians; and there was a great cry in Egypt; for there was not a house where there was not one dead." (Exodus 12:30)
These scriptures do not say whether they were male or female. However, most accept that the first born means the male child of a family who was the one who would inherit his fathers inheritance. The firstborn son held an honored position in the family and was the one who succeeded to the headship of the household. He inherited a double portion of the father's property. firstborn sons among the Israelites were those in line to become the heads of the various households, they represented the entire nation. commanded that "every male firstborn that opens each womb among the sons of Israel, among men and beasts," be sanctified to him. (Ex 13:2) Thus, the firstborn sons were devoted to God.
The “first-born” means prime, most excellent, most distinguished and in reference to Christ would be understood to be, not part of creation, but the most distinguished in relation to those who were created. The first born held a special position.
The only reference I can find in the Bible of the term referring to a female is the apostle Paul when he speaks of the followers of Jesus Christ who have been enrolled in the heavens as "the congregation of the firstborn," the first ones accepted by God as sons on the basis of their faith in Jesus' sacrifice and the first of Christ's followers to be resurrected with no need to die again.(Heb 12:23)
However, I cannot find any reference of the term "first born" to a female, thus it seems most likely that those "first born" killed in Egypt were males. This would interpret the phrase in the analogy of the faith and linguistically in the most common way the phrase is used.