I own five books on Firefox: Firefox Hacks, Firefox and Thunderbird Garage, Firefox Secrets, Firefox and Thunderbird (beyond Browsing and Email), and this one: Firefox for Dummies.
Although I like to have more than one book on a particular subject I can confidently say that - for me - this book alone makes the other four superfluous.
It describes all the essential parts in enough detail and in simple language that every non-geek can understand. It also includes a chapter on printing, which is ignored by the other books. Furthermore, the treatment on security, privacy, downloading, customization and extensions is also much easier to follow.
In short, if you only want to have just one book on Firefox, make it this one.