Nobody is preventing her from speaking. Her employer just decided not to continue to employ her. That's their right. And by the way, urging mass murder is a pretty ridiculous thing for an employee to do. I'd have fired her myself if she worked for me.
There you go / you just admitted you really didn’t take issue about her right to speak. You assumed she was going to commit mass murder by a thought. A thought before any action...hello... is the way that I see this. Free speech is not all about feel good words. I wouldn’t want to work for a person as you because you certainly would fire me for posting my thoughts on this very issue. (when I would say to the boss, she has a right to say that!)