Going after you with a rifle is illegal in its own right. The first amendment is not a factor in any way.
Firing you for it is unambiguously and unconditionally legal, unless you're in a state that has other limitations on your ability to terminate employees.
There is no scenario you can contrive where a non-government employer firing an employee for speech can be connected to the first amendment in any way. The first amendment can only possibly be relevant to the government.