Using Extension source viewer for Chrome, Firefox, and Opera is simple: click the button, and two actions become available: Download the extension as a zip file and View Source.

The default action (showing the above options) can be changed via the "Primary action on click" menu when you right-click the extension button.

Selecting the "View source" option opens a new tab with a simple viewer with some useful features. At the upper-right corner of the viewer, you can find the options to download the source code as a zip or crx file.

The viewer shows a list of all the file names, and you can filter them by using a regular expression. You can also search for specific files that contain a particular string or match a specific regular expression.

The viewer allows you to quickly jump between search results or from/to a specific line. It also automatically beautifies the code and provides syntax highlighting for better readability.

Additionally, the viewer provides an image preview and shows hashes (md5, sha1, sha256, sha384, sha512) of the file content. You can view the contents of embedded zip files and download Chrome Web Store extensions for a different platform (Chrome OS or NaCl). You can now easily view the contents of any URL or zip file. You can also get a permalink to a file and search results within an extension file.

Extension Source Viewer is automatically enabled on the Chrome Web Store. Additionally, it allows you to view the source code of extensions for Chrome, Opera 15+, and Firefox hosted outside their respective stores. You can also display a context menu entry on links leading to a Chromium extension. These features can be turned on or off from the options page.