It covers the new features of Firefox 3 from the regular user's & Developer's perspective. At just 30 pages it is a quick read. The author thankfully does not eulogise Firefox. Read along with the tips & tricks on the Firefox website, it's a good introduction to Firefox 3.
There are many new changes that make for a good reason to upgrade to Firefox 3. I liked the fact that the IE 7 feature of enabling / disabling Flash on the fly is possible now from the menu option Tools > Add-ons | Plugins. This is good if you are visiting websites that have pesky Flash based ads. Used along with the "Block images from .." option, we can get rid of irritating image ads originating from parasitic websites as well.
There is a noticeable performance improvement over Firefox 2. There are other interesting bells & whistles like tab cloning (drag a tab holding Ctrl to copy it instead of moving it), resizable search bar (drag the separator between the address and search bars), One-Click Bookmarking
Firefox 3 supports CSS 2.1 & Animated PNGs (APNG) natively. I liked the author's explanation of APNG
If you were browsing the web in the nineties, you certainly remember those cute but very annoying, poor quality GIF animations that plagued the Web. They only had 256 colors, and they didn’t really support transparency. More recently, they have been rediscovered as throbbers for almost any Ajax web application: whenever you have to wait for the end of an XMLHttpRequest call, chances are that you’ll see a throbber spinning to indicate that you have to wait a little longer.
Although GIF animations look ugly for anything more complex than a throbber, APNG images do not. APNG images are definitely smoother, and they support transparency and 24-bit colors.
.. APNG is fully backwards compatible with PNG, so any browser that's able to display PNG correctly (or incorrectly, like IE6 does), will render an APNG animation as a static PNG.