Nothing you say here makes me think you have been defamed. "It's not working out" is a perfectly good reason to fire an at-will employee. Assuming you were an at-will employee without a contract, and assuming they fired you for reasons other than illegal discrimination, I see no wrongful termination claim or defamation claim here.
Don't take the above as legal advice because I'm not licensed to practice in Washington. It's just information based on general principles of law which is intended to educate. If you need legal advice, please consult a lawyer who holds Washington licensure.
I am licensed in Washington and I agree with Mr. Taylor. You don't have a defamation suit. However, you could (read should) check with your previous employer and ask if it would be accurate to state on future applications that you were simply let go due to their lack of continued need for your services, aka laid off. Fired has such a negative connotation and it may not be the case that you were really "fired." It can't hurt to ask them if they would object to such a characterization.