Genesis 9:18 The sons of Noah who came out of the ark were Shem, Ham and Japheth. (Ham was the father of Canaan.)
Later, Noah cursed Canaan but blessed both Shem and Japheth. His blessings were not exclusively for the firstborn.
Isaac, who had a taste for wild game, loved Esau, but Rebekah loved Jacob.
Isaac loved Esau so much that he wanted to give him all the blessings.
After Jacob took the blessing deceitfully,
Genesis 27:36 Esau said, “Isn’t he rightly named Jacob? This is the second time he has taken advantage of me: He took my birthright, and now he’s taken my blessing!” Then he asked, “Haven’t you reserved any blessing for me?”
Gensis 27:37 Isaac answered Esau, “I have made him lord over you and have made all his relatives his servants, and I have sustained him with grain and new wine. So what can I possibly do for you, my son?”
38 Esau said to his father, “Do you have only one blessing, my father? Bless me too, my father!” Then Esau wept aloud.
Esau didn't think that there was only one blessing exclusively for the firstborn.
In Genesis 49, Jacob blesses all his sons.
28 All these are the twelve tribes of Israel, and this is what their father said to them when he blessed them, giving each the blessing appropriate to him.
It does not seem that only the firstborn was to receive a blessing from their father before he died.
16 that there be no sexually immoral or godless person like Esau, who sold his own birthright for a single meal. 17 For you know that even afterward, when he wanted to inherit the blessing, he was rejected, for he found no place for repentance, though he sought for it with tears.
Esau sold his birthright, in a sense, fair and square. He couldn't really blame Jacob for that. Afterward, he kept his eyes of the blessing which he knew would be big because Isaac loved Esau more than Jacob. Esau had placed more value on the blessing than the birthright. He saw them as separate things. He belittled it over losing the birthright but cried over losing the blessing.
Genesis 25:34 Then Jacob gave Esau some bread and some lentil stew. He ate and drank, and then got up and left.
So Esau despised his birthright.