Can I be fired for something I posted on social media?

The answer generally is yes. Most employers can fire you for your social media posts, even if they don't pertain to work, as most employers can deem that those comments go against their own moral or ethical code, can harm the business in some way, or they simply just think that it could add to a hostile work environment in their workplace, depending on what you shared, what your opinions are. The only real exception to that is if terminating you for your posts would rise to a level of some retaliation at work. Maybe you're complaining about something somewhat work-related, could have been racial or gender-based, and therefore them firing you would be some form of discrimination. I think those would be rare instances, but that's also possible. I think best course of action is to assume that your social media posts can get you fired, and you should be careful about what you post.