I think Yuichiro Kobayashi gets the crown for the first Firefox 3 book published anywhere. Kobayashi-san’s できるポケット+ Firefox Mozilla Firefox 3対応 (Amazon Japan no-affiliate link) book (it is basically translated as “Mozilla Firefox 3 pocket guide”) is out now will be published in early August by Impress Japan.
This is a book for beginners to Firefox and goes over how to install Firefox, how to use many of the features of Firefox, and goes over 20 popular add-ons in detail. It’s a full-color book and is quite small (maybe paperback book-sized.)
O’Reilly Japan is also planning to publish a Firefox 3 Hacks book later in August which I will certainly blog about here when I have something to show.