My boss sent out a letter to all employees criticizing my work. I went in to talk to him about it and he fired me. Now he's saying I quit, Can I do anything?

You need to immediately send out an email to your boss confirming what had happened and that he fired you on the spot. You should say in your email that if your understanding of what happened is not correct, you are happy to come back to work.

If he does not allow you to come back to work, that could be considered a termination, which means you get severance. This needs to happen right away (ie this morning). It is very important that you also call me to discuss.