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The tort of breach of fiduciary duty is something that corporate officers can be sued for, if they are not loyal to a corporation. Shareholders can sometimes be sued as well, especially if they do something to affect the value of the stock.
Sometimes, employees can be sued for breach of fiduciary duty because, for example, they take confidential information to a competitor; or they work for a competitor at the same time. Employee breach of fiduciary duty is usually associated with "key" employees who hold a position of "trust and confidence."
Because you were an accounting coordinator; you were not a corporate officer (and probably not a shareholder); and you had never filed that company's taxes before and were not asked to do so, I don't think that you would be considered as having been in the necessary position of "trust and confidence".
I don't think that if they did sue for "breach of fiduciary duty" that the company would win. Do keep in mind, though, that people file meritless lawsuits all the time -- so you should talk to an attorney in your area (you will need a business attorney) if they continue to threaten you.