1. Drag this link to your bookmarks bar: Utility
2. Run python -m SimpleHTTPServer from your shell in any directory containing a file named utility.js. You may need to install Python if your system doesn't have it.
3. Visit any page in your browser, and click the Utility link from the bookmarks bar to run your utility.js script.
4. If you see the checkmark in your JavaScript console then you're good to go!