"At will" does not always mean that an employer can dismiss you for a bad reason - like to prevent your participation in a retirement plan, your age, your disability. I am kind of late to jump in but you might want to talk to an employment attorney if you think that you were dismissed for a bad or improper reason. Not sure of CA law but in AZ - which is also an "at will" state - an employer cannot dismiss you for an improper reason.

Good luck - sometimes employers do us a favor when the dismiss us - another door opens.

Take care.