Nothing quite says, "Happy Birthday!" like "You're fired; I never want to see you again!" When you get fired, there's a lot of pain and worry attached to it. You might not have a ton of money saved up during that crazy 1-6 month period of job hunting. That being said, getting canned can make you feel liberated and relieved that you're never going to have to deal with that specific group of people ever again. They've cut off the working relationship, and all you can do is go without a fight and find a new way for yourself.
One worker recently went viral on Reddit after he received a birthday card in the mail from the company that had just fired him. Now, this might seem like an honest mistake, with the birthday unintentionally overlapping with the firing, but he had been canned three weeks before the note was sent, which really rubbed salt in the wound.