OpenAI staff have signed an open letter threatening to quit unless the board resigns and reinstates Sam Altman as CEO, following his controversial firing on Friday. Over 730 employees, approximately 95% of the company, have signed the letter. The staff are considering joining Altman at a new Microsoft subsidiary, which has offered positions to all OpenAI employees. The incident has highlighted OpenAI's unusual governance structure and the tensions between the company's mission and commercial goals.