The Firefox browser itself provides a robust extension framework, allowing developers to conveniently modify and extend its behavior. Many websites, such as del.icio.us, offer Firefox extensions to provide a better user experience. Acquiring knowledge in this area is not only useful for web developers but also allows others to solve specific problems encountered while using Firefox by developing their own extensions. This article uses a Firefox extension capable of batch-downloading all images from a given HTML page as a case study, providing a detailed introduction to the development process of Firefox extensions. This includes setting up the development environment, using XUL to describe the user interface, employing JavaScript to add functionality to the extension, as well as aspects related to packaging, publishing, and updating the extension.