Extension Source Viewer is a Firefox add-on designed for developers and IT professionals to easily access the source code of browser extensions. It provides a convenient button next to the address bar that activates when a compatible add-on is detected. Users can choose to download the extension as a zip file or view its source code directly. The primary action can be customized, enhancing user experience based on preferences.

The viewer includes a range of robust features such as automatic code beautification, syntax highlighting, and a file filtering system using regular expressions. Users can quickly navigate through search results, view embedded zip file contents, and even obtain hashes of the file content. It also supports downloading extensions from other platforms and provides a permalink feature for easy reference. This tool is essential for developers looking to analyze or modify browser extensions.