Firefox Developer Tools is a set of built-in features in the Firefox web browser that helps developers inspect and debug web pages. It includes tools for examining HTML and CSS, monitoring network requests, and testing JavaScript performance. These tools allow developers to see how their code interacts with the browser in real-time. With Firefox Developer Tools, users can also simulate different device screens and view responsive designs. The console feature helps in logging errors and running JavaScript commands directly. Overall, these tools enhance the web development process by providing essential insights and functionalities.