The top of Mount Gerizim - The ancient Shechem was perhaps situated there. The population of Shechem is supposed to have been keeping some public festival outside the city when Jotham addressed them.
Related Commentaries of Judges 9:7
Judges 9:7
7 And when they told it to Jotham, he went and stood in the top of mount Gerizim, and lifted up his voice, and cried, and said unto them, Hearken unto me, ye men of Shechem, that God may hearken unto you.