Final answer:
A firm should stop hiring more employees when the market wage equals the marginal revenue product.
Step-by-step explanation:
The profit-maximizing level of employment for a firm is reached when the market wage equals the marginal revenue product. The marginal revenue product is the additional revenue generated by hiring one more worker. When the market wage is equal to the marginal revenue product, hiring more employees would result in higher costs without generating enough additional revenue to justify it. Thus, a firm should stop hiring more employees at this point to maximize profits.