As a general rule, unless you are in a union job, you can be fired for any reason or no reason at all in WA. The only exceptions are that you can't be fired for an illegal reason (i.e. because of your race, gender, nationality, sexual identity, age, etc.). You also have certain protections against retaliation for engaging in certain behaviors (whistleblowing, union organizing, other...). But filing a lawsuit against a former employer is not protected activity and could hypothetically result in your termination from your current employer.