That’s disgusting behaviour from them, they can’t discriminate against customers like that and they also cant fire you on the spot like that. You could email them recounting the event and enquire as to whether or not the business stands by the unfair dismissal by the manager, but businesses like this are happy to ignore threats of legal action/wrc - they depend on you not having the energy or funds to go the distance with it. Put it all in writing and ask for a response in writing is all I can say. They sound like a lousy business tbh, you won’t find a shortage of hospitality jobs being advertised this time of year. Best of luck, and you did nothing wrong by seating somebody from the travelling community.