Question
Why in Jeremiah 31:9 is Ephrayim referred to as G-d’s first born?
Answer
Ephrayim wasn’t even Joseph’s first born; his brother Menasheh was. But in Ch.31 Jeremiah is prophesying that the Northern kingdom (the Kingdom of Israel) would return from exile and come back into G-d’s favor. Ephrayim is named because the tribe of Ephrayim played a leading role in the Northern kingdom. He is called "first-born" to indicate that "he is as beloved to G-d as a first-born son is to his father" (Commentary Metzudas Tzion on v.9); see also v.19.