Can you be fired for blowing the whistle on your employer's illegal actions? Of course. You should expect it. Is it legal to fire you for that reason? No. You would have the basis for a wrongful termination suit.
My suggestion...consult with an experienced employment law attorney in your area, who could steer you in the right direction, as far as to whom to report your employer, and how to best protect yourself in the process.
They say you get what you pay for, and this response is free, so take it for what it is worth. This is my opinion based on very limited information. My opinion should not be taken as legal advice. For true advice, we would require a confidential consultation where I would ask you questions and get your complete story. This is a public forum, so remember, nothing here is confidential. Nor am I your attorney. I do not know who you are and you have not hired me to provide any legal service. To do so would require us to meet and sign written retainer agreement. My responses are intended for general information only.
Sadly, this is a practice that happens ALL THE TIME in California. My offices is representing a Plaintiff who is suing a restaurant that ONLY hires workers who are in the USA illegally --- so they can fire them and not pay overtime. The minute a worker is injured -- the worker is fired!
If you are brave, and willing to report this, make sure your report is to a GOVERNMENT AGENCY, and it is VERY IMPORTANT that you keep good records to prove you made the report. If your employer fires you after you report something like this TO A GOVERNMENT AGENCY you will have a lawsuit for "Wrongful Termination in Violation of Public Policy" -- and perhaps more.
If you decide to get an attorney, don't allow yourself to be referred to a Worker's Compensation attorney. This is a different kind of case. You need a referral to an employment discrimination/wrongful termination lawyer.
If you are fired for reporting illegal activity to a government agency, you would have a claim for wrongful termination under the "whistleblower" statutes, which include California Labor Code 1102.5, among others. If this happens, you should call an attorney, immediately to protect your rights as there are strict time deadlines on the filing of these types of claims.
Nothing in this communication should be construed as creating an attorney client relationship. This is for informational purposes only. Attorney will take no action on your behalf unless and until a written retainer agreement is signed. There are strict time deadlines on filing claims and, as such, you are advised to consult with and retain an attorney immediately to file such claims timely or you will lose any right to recovery.