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You don't have to provide your tax statements to anyone in CA State Court litigation (unless they can prove that your financial assets are actually at issue in the litigation - which is very difficult to prove, even if you are a party to the action, which you are not).
This protection is so strong it is actually provided for in the CA State Constitution.
You can politely but firmly write back that you are declining to disclose your financial information as protected under the CA State Financial Privacy Laws.
If the matter goes to litigation, you will need to respond more formally, but the matter is still pretty clear - you can find a fairly detailed discussion of the legal authorities in this outline: http://california-discovery-law.com/privacy.htm
I would still probably assert the same protection (especially if you are outside of litigation).
If the parties are actually in litigation - and this is a subpoena (not just a "friendly" letter request from one of the attorneys), then consider filing a "motion to quash" the subpoena.
There is no reason why an accountant's client's tax records should be part of the litigation in an accounting firm's break up.
(Obviously I don't have all of the information on your case, or what is going on here, but it would be quite a stretch as to why they need your documents, and you can definitely push them to explain why they should receive your company's tax records, and then how they plan on protecting those documents from disclosure in public in the litigation (everything filed in a court case is part of the public record, and as I am sure you are well aware, promises of "in camera review" or similar privacy are well and good until someone decides otherwise)).
Unfortunately, to file the motion to quash, you will need your own civil litigation attorney (corporations cannot appear "pro per"), but fortunately, this is a relatively simple and inexpensive matter to do (any civil litigation attorney should be able to do this standing on their head).