Esau despised his birthright. In order to understand this, first we need to understand what it means in this context to "despise" and also what the birthright was.
What was a birthright?
An abundance of sources will tell you that in the Ancient Near East context, the birthright goes to the eldest son. All sons receive a "portion" of the inheritance of ...
I think you've mostly answered your own question. Esau is the father of the Edomites and Amalek (grandson of Esau) is the father of the Amalekites.
Genesis 36:9 ESV These are the generations of Esau the father of the Edomites in the hill country of Seir.
Genesis 36:12 ESV Timna was a concubine of Eliphaz, Esau's son; she bore Amalek to Eliphaz.
Esau was the progenitor of the Edomites. Pretty straightforward.
Now whilst Amalek was indeed the grandson of Esau, who later had children of his own called the Amalekites, there existed another "Amalek".
These Amalekites were one of the many ancient inhabitants of the Levant and when the word "ancient" is used in Old Testament, it REALLY means ancient. ...