I keep reading that an employer can fire an employee for any reason, at any time, as long as the firing is not based on age/sex/race/religion.

I'm asking because I don't know the legal history, but how frivolous can their justification for firing someone be?

As frivolous as for no reason whatsoever.

As frivolous as for no reason whatsoever.

Villian is correct. Employers can fire for any reason or no reason, so long as the reason is not prohibited by law.

I could wake up tomorrow and decide to term an employee who has been a steady and faithful employee for 15 years, just because I was in a bad mood and decided I did not like him. It would be a legal termination. It would also be stupid, but laws don't exist to protect businesses from making stupid decisions.

I keep reading that an employer can fire an employee for any reason, at any time, as long as the firing is not based on age/sex/race/religion.

I'm asking because I don't know the legal history, but how frivolous can their justification for firing someone be?

California is better than most states when it comes to wrongful termination suits. So if you have long faithful service,and the boss does something as stupid as fire someone with 15 years service because he woke up that morning and figured he did not like the guy. He would have a good chance of ending up with a wrongful termination suit in California,on several theories of law.
So to answer your question no in California you can not be fired for a frivolous reason,the longer you have been employed with the company the more difficult that would be. The employer may think that but the courts wont always see it that way. Keep in mind also that wrongfull termination suits are not enforced by the state D.O.L.
JoeC

I'm not sure if this follow-up is in the scope of these forums, but I was curious, how would one prove that they were fired wrongfully?

For example, say I work at a department store, and my manager tells me to clock out, but finish up what I'm doing afterwards, since he would get in trouble if I work over 8 hours. Whatever I do it. Then someone else comes along, and he does not do it. Two weeks later, the manager fires him.

This hasn't happened to me or anyone I know, but I have heard stories.. and I always thought that employees had tons of rights (until I came to these forums), so I'm curious. These forums are simply amazing for me =b

Given that scenario the first employee is first of all a stooge. The employee that refused,and was terminated would possibly have a case for wrongful termination,under violation of public policy. The overtime laws are public policy the employee exerted those rights,and was terminated. You can not terminate in Ca when the employee adheres to public policy.
Now comes the hard part EVIDENCE,if employee can get the stooge employee to admit that is truly the case. Or even more remote get the employer to admit that he violated public policy then he could win.
Violation of public policy wrongful termination suits are complex,and would require a lawyer. If the damage remedy is small finding an attorney may be difficult.
JoeC