Sforno on Genesis 49:3/ספורנו על בראשית מ״ט:ג׳

1יתר שאת ויתר עז, פחז כמים אל תותר; you are as unstable as water; you will therefore not enjoy an advantage over your brothers, the advantage which would have consisted of יתר שאת, enhanced social position and the entitlement to the firstborn’s extra share in the inheritance as well as the status of priesthood. Even though you are biologically, ראשית אוני, and as such would have been entitled to privileges involving an extra share in my inheritance, (based on Deuteronomy 21,17) as well as

2יתר עז enhanced political clout by supplying the kings of the Jewish nation from your tribe, you have forfeited all this by having displayed character weakness. (compare Chronicles II 21,3 where the claim to the throne by Yehoram was based on his being the firstborn).