There is a second interesting point in Jeremiah 31.9b alone. Again, it reads “For I am a father to Israel, Ephraim is my first-born.” What’s so interesting about that? Well, it’s interesting because in the actual story line of the Old Testament Ephraim was not a first-born son.
Joseph has two sons in Egypt, whom we consistently refer to as Ephraim and Manasseh; however, we should in reality refer to them as Manasseh and Ephraim. Manasseh was first-born and Ephraim second. The reason we do not use this ordering is that this story is one of those that reflects God’s preference for the younger son in the Old Testament. The younger son receives Jacob’s blessing in Genesis 48, just as Jacob as a younger son had received the blessing of his father Isaac in the place of Esau. And, this is just as Isaac as a younger son was the heir to the promise to his father Abraham in the place of his older brother Ishmael. And, so on and so forth. For a more thorough list of the stories that reflect this theme check out this post: Younger Sons who Rise to Prominence in the Bible.
Interestingly, there are a number of different proposals for why this preference makes its way into the Old Testament stories. They range from the fact that this is common theme from the realm of fairy tales to the idea that these stories are pro-Davidic, i.e. since David was a younger son there needed to be some precedent for his being chosen as king. This material provides for interesting study, but since some of the material is fairly spread out many do not realize just how prominent the theme is.