This book takes a "do it yourself" approach when it comes to writing code, because the author firmly believes that the best way to really learn a programming language is by writing code. So there's no cut-and-paste-ready code downloads, which you can simply put in your pages. On the contrary, you're expected to type in code, see how it works and then tweak it and play around with it. When trying out the code examples, you're encouraged to enter the code into Firebug's console. Let's see how you go about doing this.
As a developer, you most likely already have Firefox installed and use it for your daily web browsing pleasure. If not, do yourself a favor and install it right now. It's free and runs on any platform—Windows, Linux, or Mac. Download it from http://www.mozilla.com/firefox/.