If I remember rightly, its a skin, but it's an official Mozilla release and I was prompted to download it immediately after I installed Beta 2. It goes under the name of 'proposed theme for Mac OS X' so it may well end up being the default look. It looks like it's under consideration judging by the developer's comments on the Mozilla site:
https://bugzilla.mozilla.org/show_bug.cgi?id=397723
Personally though, I quite like it (more than the old look). It in some ways its strikingly similar, but subtly different to Safari. Anything that conforms to the standard interface guidelines to give a consistent look across all applications is a positive thing in my opinion. I guess there are only so many ways to present the forward/back/reload/stop/home buttons if you're trying not to break the Leopard look and feel.