He has freedom of speech as an individual. If he had made those statements in association with being a firefighter, the county could fire him on the basis that his speech bears the imprimatur of the state. (This means that people could associate his speech with the government). But he made those statements as an individual so firing him would be a violation of free speech.
Someone like Rosie O'Donnell on the other hand did get legally fired, because she used her network as an outlet to put out her opinion. In doing so, it made it appear that she was speaking on behalf of NBC. Under these circumstances she can get fired.
Then again our country is fucked up and the dude will probably lose his job anyway.