Firebug is the best way to catch our JavaScript errors; it can also give us short cuts to seeing the included JavaScript files.
Open a page that has one or more JavaScript files included via SCRIPT tags. The code snippets in the book include one for just this purpose, though, any of them will do.
Open the Firebug window and click the Script tab. The horizontal bar beneath the tab list will have a drop-down that shows the name of the URI, for instance, 09-05.html. This drop-down menu will have a list of each script that is included. Visible in the list, if viewing the scripts for 09-05.html, will be 09-05.html, mootools-1.3.0.js, and mootools-more-1.3.0.js.
Clicking on any of these files will load the source into the panel for review.