The first amendment's "free speech" means that the government isn't able to charge you for what you say (unless it's like a death threat, or hate-speech)
The right to free speech is not applied in any other circumstances. It's not about your workplace, it's about the government.
You do not have the right to say whatever you want at work. They could even make a policy that saying "thank-you" was against the interests of the company, and fire you for saying "thank-you" too often.
People from other countries should not know the laws of your country better than you do.