This add-on injects JavaScript into mozilla.org web pages.

This extension includes a content script, "borderify.js", that is injected into any pages under "mozilla.org/" or any of its subdomains.

The addons.mozilla.org domain doesn't allow scripts to be injected into its pages. Therefore, this extension doesn't work on pages in the addons.mozilla.org domain.

The content script draws a border around the document.body.

From this example, you see how to inject content scripts declaratively using manifest.json.