A Greasemonkey developer creates user scripts to modify the appearance and behavior of web pages. Using JavaScript, they enhance user experience by adding new functions, automating tasks, or changing the layout of websites. They may also fix bugs or remove undesirable features on a webpage. Greasemonkey developers often create custom scripts for personal use or share them with the online community. They need to understand browser compatibility, web security, and DOM manipulation. Furthermore, they use Greasemonkey, a Firefox extension, which enables them to install and manage these user scripts effectively.