Hi there. I think I have an answer to my own question about firstborns. "The firstborns are a reminder of the Lord's ownership of Israel, the people he calls his firstborn sons. It is also a reminder that the Lord provides for and protects his firstborn son." This is much like the story of Abraham and Isaac where God provides a ram in place of Isaac.
I did some searching online for an answer or deeper interpretations of Exodus 13:11-16. And I found it in the following website. Female firstborns were never mentioned as the firstborns were a symbolic representative of the Lord's claim on all firstborns.
http://livingwordrec.ca/archive/firstborn/
Regards,
Mandy