Is Friday prayer valid if the worshiper performed ghusl from masturbation before the prayer, then found drops of urine on his underwear and penis after returning from the prayer, and was not aware of their presence?

What you saw of urine after purifying yourself and praying has no effect, and your prayer is valid; because the impurity is attributed to the nearest time of its occurrence, so the default is that it exited after the prayer.

And the prohibition of masturbation is not hidden.