I'm making this post from the Bon Echo Alpha 2 build of Firefox. The greatest new feature by far is the inline spell checking feature for
textarea elements. It works exactly like the spell check feature in Thunderbird, and it makes blog posting that much easier.
The new tab UI is nice, but I haven't yet opened a ton of tabs to see how they handle overflow and the like. I also like the new "Addons" window, which is essentially the marriage of the "Themes" and "Extensions" windows from current Firefox releases. Other cool features include an improved default search box (although Googlebar Lite is still the greatest one around ... ;) ), session saver (in case of a browser crash), greatly improved feed discovery capabilities (I really like this), and more. All in all, Firefox 2 is starting to look really good. I can't wait to get my hands on a more mature release; I will definitely make the switch once the public betas arrive.
The only gripe I have is that the few extensions I have tried to install all fail (the maxVersion isn't set high enough), so I'm currently extension-less. :(
On an entirely unrelated note, I have added a new Extensions category here to this blog, and I have placed all Firefox extension related discussions under that category.