What is the ruling on praying behind an Imam who prolongs the sermon and insults people, and is it permissible to go to another mosque if there is no third mosque available?

If the Imam curses people and burdens them by prolonging the sermon, then there is no harm for the congregants to perform the Friday prayer in another mosque.

It is better for a person to pray in the mosque of their neighborhood, unless there are violations or deviations in that mosque that necessitate moving to another mosque. The Imam should be advised and called upon to shorten the sermon and consider the condition of the congregants.

Sheikh Ibn Uthaymeen, may Allah have mercy on him, was asked about going to another mosque because its preacher has extensive knowledge and good delivery. He replied: "It is better for the people of a neighborhood to pray in their own mosque for the sake of acquaintance and harmony, but there is no harm in going to another mosque for a religious benefit, such as acquiring knowledge or listening to a more influential and knowledgeable sermon." The Companions used to do this, such as Mu'adh ibn Jabal.