What is the following new theory called undwhich fir couldbe held liable if their stock prices fell?

The theory you are referring to is likely the "duty of care" or "duty of loyalty" in corporate governance, where shareholders might argue that a company has a responsibility to act in their best interests. If a company's stock prices fall due to perceived mismanagement or failure to disclose important information, shareholders may seek to hold the company's executives or board members liable for damages. This concept relates to potential legal actions under securities law, such as class action lawsuits. However, the specific name of the theory may vary based on context and jurisdiction.