Back in 2005, I've read "Rapid Application Development with Mozilla" by Nigel McFarlane - pretty big and comprehensive (I think) book about Mozilla platform. I'm not a web developer but I always try to keep up with the latest developments in the industry, so I was looking for a book which covers Firefox peculiarities. That was my primary motivation for buying "Programming Firefox" by Kenneth C. Feldt.
The book itself was a bit disappointment: it was not as comprehensive as I expected it to be (honestly, I thought it would be both comprehensive and slim - probably I was just asking for too much). The author employ "learning by doing" approach - in the first half of the book he develops a XUL application and explains things required to build it. Although this approach works really well for the magazines where you are limited in space, when you read a book you expect more general discussion which is applied to the specific subject only at the very latest step. If you are trying to explain everything using only one (or very few) program(s) as a test base you will necessarily limit the discussion to topics relevant to that problem - which is again okay for the magazine but not for the textbook. The most notable omissions from the book in this sense are: whole XPCOM framework (several interfaces and the way to create the components via XPConnect are mentioned briefly but it does not go any further solving problems relevant for the example application); keys, keysets and commands (nothing was said about it), Mozilla-specific CSS attributes (-moz-appearance is mentioned briefly), complicated layouts (deck, stack and the friends). I would also prefer to see brief introduction to JavaScript.
A strong part of the book is that it's not limited to XUL/XBL - SVG, XForms and canvas tag are also mentioned. Unfortunately (and again), only XForms are covered throughly - when it comes to SVG you see small subset of features required to build bar diagram.
As mentioned in the previous review, code is formatted badly (and actually I can object some of the techniques employed therein) - but I don't think its a big drawback of the book. The worse thing is that there is too much code - not only snippets, but the whole programs. They take up precious place ans are hard to read from the paper anyway.
In the conclusion, it's not a brilliant book on the subject. Given the absence of the modern books on XUL programming and its relatively small size in terms of pages, I recommend you buy it if you want to get a taste of Firefox development, but if you need an old-school textbook, better opt for "Rapid development..." or whatever. I'm giving it four-stars anyway - there is no point for being too strict to the author and the publisher who are willing to promote good under-documented technology.