“Do not curse.” This can be an actual curse, or simply making light of the person, speaking in an unsupportive way.
“not in your thoughts.” What people think often comes out in unguarded moments. There is a possibility that the Hebrew text could read something such as “not with your friends” (cp. JPS Bible Commentary by Michael Fox, who translates the verse, “Don’t revile a king even among your intimates”).
“do not curse the rich.” The tie in between Ecclesiastes 10:19 and 10:20 could be that money helps with everything, and in the ancient world, money was often tied to who you knew and your connection with them. If the king of the rich people felt you did not support them they might cut off your source of money.
“the air.” The Hebrew uses the standard phraseology for air: “bird of the heavens.”