Most private employers have rules about conduct. 1st amendment rights have nothing to do with those rules even though many people incorrectly interpret them that way. Your employer can fire you for things you say or do at work. They can also fire you for things you post on social media outside of work hours.
They key to the 1st Amendment is "Congress shall make no law." Trump saying sonuvabiches should be fired, or employers actually firing you for things you say or do is in no way threatening 1st Amendment rights.
1st Amendment - Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.