You are wrong, but in the right general direction. "Laid off" means involuntarily terminated because of a reduction-in-force / elimination of positions. "Fired" means involuntarily terminated for any other reason. Except in the case of positions with special protections, either contractual (personal or union) or legal protections of the type that apply to career civil service positions, you cannot assume firing is about either misconduct or performance; it can just be because the boss decided his nephew needed a job.

And "let go" subsumes all forms of involuntary termination, both layoff and firing.