A browser extensions developer designs and builds small software modules that add functionality to web browsers. They use various programming languages like JavaScript, HTML, and CSS to create features such as ad-blockers, password managers, or themes. Developers must ensure compatibility with different browsers and follow their specific guidelines. They also debug and update extensions to maintain performance and security. In addition, they often work with user feedback to improve the extension. The goal is to enhance user experience by customizing and extending the browser's capabilities beyond standard web pages.