A Firefox extensions developer designs, builds, and maintains browser add-ons that enhance Firefox's functionality or user experience. They utilize technologies like JavaScript, HTML, and CSS, and work with Firefox's WebExtensions API to implement features like blocking ads, managing cookies, or integrating with other services. They also debug and fix issues, ensure compatibility with different Firefox versions, and comply with Mozilla's add-on policies. Developers interact with users for feedback, troubleshoot issues, and update extensions to improve performance or add features. They play an essential role in customizing and optimizing the Firefox browsing experience.