If you were terminated because you complained about the use of illegal pesticides in products, you may have a claim. However, if you were let go solely because of a dispute with another, whether true or not, would unlikely provide you with grounds for a claim.
If you worked for a private company and were not in a union, you almost certainly were an "at-will" employee, in the eyes of the law. At-will employees can be terminated by employers at any time for any reason, even unfair or unfounded reasons. But they cannot be fired for a reason specifically prohibited by statute, most often involving discrimination due to race, gender, age, etc. It is also unlawful to fire an employee for being a whistleblower. So, if you have complained to an agency like Cal OSHA about the company using unsafe chemicals and were fired for it, that is illegal.
It is not clear from your post what the employer's use of copper sulphate has to do with your termination. But if you were fired because another employee complained about you, even if unfounded, that are no grounds for a wrongful termination case.
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.
In California, an employer is prohibited from retaliating (in the form of termination or demotion) against an employee for reporting or refusing to take part in that employer's illegal activity. You say that you witnessed your employer putting pesticides in the product but do not say whether or not you reported such illegal activity to an outside agency or even your employer for that matter. From the facts, it looks like you were terminated for allegedly saying something inappropriate at work and not because you spoke up about the copper sulfate. More facts are needed here to determine if this was a retaliatory wrongful termination.