Firstborn is reference to the first of the mother's offspring. It stands figuratively for that which is most excellent. The firstborn male of the family carried certain familial rites and privileges (Gen. 27:1-29, 48:13-14) and was given a double portion of the inheritance (Deut. 21:17).
The term is also applied to Christ as the pre-eminent one and the first one raised from the dead (Col. 1:15, 18). It does not mean first created as Jehovah's Witnesses believe. In fact, the firstborn rites were transferable as with Esau and Jacob. Compare Jer. 31:9 with Gen. 41:50-52.