Some developers work on Mozilla to improve the way it functions and other developers use Mozilla to create new applications. These two approaches reflect the dual nature of Mozilla as a development project and a framework for creating applications, but the line between the two isn't always clear.
People often start developing an application with Mozilla and then notice a way to make Mozilla itself work better, which will in turn make their application work better. In these cases, the developer works on both Mozilla applications and the Mozilla development framework that provides the plumbing those applications run on top of.
This distinction between platform and applications is important. This book provides in-depth information about application development using Mozilla, but it does not directly describe anything relating to the development of Mozilla itself.
If you are interested in learning how to become a Mozilla developer (and actually hacking the code), we can suggest a couple of starting points. Getting Your Work Into Mozilla is an article written by two authors who chronicled their own experiences about becoming a part of the Mozilla community, and is available at http://www.oreillynet.com/pub/a/mozilla/2000/09/29/keys.html. There is also great information about getting started with development on Mozilla from the mozilla.org site at http://www.mozilla.org/get-involved.html.