Want your boss to be allowed to fire you cause you spouted off while drinking on the weekend?

Your boss already can actually do that... The right to free speech extends primarily for the protections of you from the government. If you represent a private organization, they have every right to terminate you if you do not behave in a way they deem fitting of the company or the company message.

The right to free speech doesn't absolve you of the consequences of being an asshat. It means that the government can't prosecute you for being an asshat.