The 1st amendment doesn't protect your job, never has.
Sorry, I’m not American so have little idea how it works.
I also did not agree against the legality of being fired for a social media post. It's been happening since social media first existed.
You also can't boycott an employee. I really don't know what point you are trying to make here.
The default employment rule is at-will, meaning someone can be fired for any reason not explicitly prohibited. Political affiliation is not a federally protected category.
Even then, in practice they can fire people for prohibited reasons, as long as it can't be proven that those reasons were used. Which in practice could be very difficult.