This is a good book for the general public on how to get the most out of Firefox. It starts out with the basics, works its way to the really useful exensions, and finally gets under the hood fine-tuning the many preferences in about:config. I would recommend it to people that want to know more about Firefox but haven't tried it yet, or people that have it and aren't comfortable tweaking it without clear directions.
Personally, I was a little put off by the author's cheerleader tone. They provided a pretty complete howto, but didn't get into missing Firefox features, the roadmap for the product. I like a more balanced tone in my technical manuals.
On the plus side, it did deal with some of the common issues and how to troubleshoot them, which is a plus for any product book going to the general public.
In all, I rate the book a 7.5 for Firefox beginners, and a 6 for experienced users.