If he was getting uncomfortable, then it is his problem. You were not causing any discomfort, you were struggling and that is okay. However, it is illegal for the company to fire you because you stutter. This is discrimination and you can definitely sue your boss for this. Your boss can get into serious trouble for discrimination against an employee basically because of gender, race, nationality, skin color, ethnicity and disability including speech disorders. I am sorry that you had to go through this and guess what, even if I was your customer, I would not feel uncomfortable or get offended if you stutter because I also stutter as well. I've been stuttering for a really long time, for years in my early teenage years, so I know how frustrating your stutter really is. Always remember that you did not make that customer feel uncomfortable, he was uncomfortable because he was really rude, arrogant and impatient with you. What your boss did was illegal, you need to sue your boss for firing you just because you stuttered.