‘And when Joseph saw that his father laid his right hand on the head of Ephraim it displeased him, and he held up his father's hand to remove it from Ephraim's head to Manasseh's head. And Joseph said to his father, “Not so, my father, for this is the firstborn. Put your right hand on his head.” '
Joseph is upset. As a father he wants his sons to be treated fairly (possibly he remembers what the result was of his father's favouritism). He is so incensed that he interrupts the blessing. Joseph's action demonstrates how important this was all seen to be. It was a matter of precedence, which was accepted everywhere in the ancient world, that the firstborn received the primary blessing, although there were exceptions. In the Keret Legend found at Ugarit it says, ‘The youngest of them I will make firstborn'. But possibly Jacob remembers his own past. He too was the younger and yet he received the firstborn's blessing. And something in his heart tells him that this is right here. It is in the end God Who is sovereign and will do His will.