The document describes a simple Firefox extension with the name "first_ext" that logs "Hello World!" to the console and then quits when run. It provides the extension's name, description, and author, and contains code to run the main function on startup which logs and quits.

20. {
"name": "first_ext",
"description": "This is my first extension",
"author": "swdyh"
}

21. exports.main = function(options, callbacks) {
console.log("Hello World!")
callbacks.quit()
}

22. (jetpack-sdk)% cfx run -a Firefox

23. (jetpack-sdk)% cfx xpi