I think it was wrong to fire her for that. Any restaurant I've ever been to where the waitstaff sings happy birthday doesn't involve every waiter in the house. She could have excused herself from it and went about doing the rest of her job. This girl who wouldn't sing, if she had any respect for the people she worked with, should have informed management before taking the job. If she didn't inform them of it, and then refused to do it, that's an indication of a disrespectful attitude toward her coworkers and manager.
That being said, I think most firings are due to "bad apples" being called out by co-workers. She probably had other behavior problems or just rubbed other waiters the wrong way, and they jumped on this one thing, being too eager to let her go. Which was really stupid on the part of the manager, and he probably lost his job, too. He should have known better. Build up a file, report her for being late, or other problems with coworkers. Give her warnings and then let her go if she doesn't turn it around.
I know an elder who got fired and claimed it was for being a witness. I didn't believe him for a second. I know this guy well enough to know he got fired for running his boring mouth all day and being lazy.
We all want to be protected from being fired over our religion or lack-of-religion, so this JW should be protected too.
As for the person who said Hitler went too soft on JWs and we should send them all to Korea? This person is in some real pain and needs to get some help. I feel really bad for anyone hurting this much. I'm sorry you are in so much pain. I know you don't mean that, or when you look at it in a few years from now, I hope you won't still be in that place.