borderify

This add-on injects JavaScript into web pages. The addons.mozilla.org domain disallows this operation, so this add-on will not work properly when it’s run on pages in the addons.mozilla.org domain.

What it does

This extension just includes:

• a content script, “borderify.js”, that is injected into any pages under “mozilla.org/” or any of its subdomains

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

What it shows

• how to inject content scripts declaratively using manifest.json