Was Esau still considered the firstborn in God's eyes?
No. Although Esau was indeed the firstborn in physical reality, so far as God was concerned he was second. He could tell Isaac he was the firstborn Esau, but the context states that from God's perspective he was not first; he was second.
Les explains that Esau answered Isaac in the same way Jacob had, speaking from the physical reality: “I am thy son, thy firstborn Esau.” He was truly the firstborn by birth order, but so far as God was concerned, he was the second. The account then shows Isaac trembling, and Les suggests Isaac feared what Esau might do. Esau is described as a man destitute of faith, and such a lack of faith affects behavior.
Genesis 27:32
And Isaac his father said unto him, Who art thou? And he said, I am thy son, thy firstborn Esau.