The First Amendment provides you with the right to free speech (among others), but it does not protect you from the consequences of that speech. If you tell the boss that you think he's/she's pond scum, don't expect the First Amendment to save your butt from being escorted off the premises with instructions to not return.
Besides, the rights guaranteed by the Constitution are, by the terms of the Constitution, applicable to the federal government and many of them, by the terms of the 14th Amendment, are applicable to the states. None are applicable to a private employer unless they've been incorporated into specific legislation.
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