Knowing how to program with JavaScript is important, but so is knowing how to debug your code. Debugging is very useful to help find bugs in your code, but it can also help you execute your code at a lower speed so you can see everything that is happening (the stack of methods called, variable assignment, and so on). It is highly recommended that you spend some time debugging the source code of this book to see every step of the algorithm (it might help you understand it better as well).
Both Firefox and Chrome support debugging. A great tutorial from Google that shows you how to use Google Developer Tools to debug JavaScript can be found at https://developer.chrome.com/devtools/docs/javascript-debugging.
You can use any text editor of your preference. But there are other great tools that can help you be more productive when working with JavaScript as well:
Aptana: This is a free and open source IDE that supports JavaScript, CSS3, and HTML5, among other languages (http://www.aptana.com/).
WebStorm: This is a very powerful JavaScript IDE with support for the latest web technologies and frameworks. It is a paid IDE, but you can download a 30-day trial version (http://www.jetbrains.com/webstorm/).
Sublime Text: This is a lightweight text editor, and you can customize it by installing plugins. You can buy the license to support the development team, but you can also use it for free (the trial version does not expire) at http://www.sublimetext.com/.