A Fuel UX developer primarily uses the Fuel UX library, a set of jQuery-based UI controls, for designing and implementing user interfaces. They incorporate components like trees, wizards, and spinners to enhance user interaction in web applications. They work on improving the user experience by integrating Fuel UX with Bootstrap for responsive design, ensuring compatibility across different devices. Their responsibilities also involve debugging and fixing issues, updating the library as per the latest web standards, and adding new features. They often collaborate with UX designers and back-end developers to create seamless, intuitive, and efficient web applications.