Firefox's upcoming Australis UI is a major revamp. It's also been in the works for a year now. It showed up already in Thunderbird, a simplified version if it, some elements are visible in the mobile Firefox design and more recently in the Windows 8 Modern UI version of Firefox 18.
But Australis is still not ready to roll out in the proper Firefox. In fact, it's not even going to be rolled out all at a time, the plan is now to implement the tab strip.
Granted, that's one of the most noticeable changes, in Firefox 19 at the earliest and then add the other features later.
If you've kept an eye on Australis development, you'll know how it looks like and, by now some features have already been enabled in various Mozilla apps.
The entire UI is going to be more fluid (i.e. it's going to have rounded corners), there's going to be a bigger emphasis on the active tab, pinned tabs will be better separated and so on.
With Firefox 19 landing in the Nightly channel next week, we should be seeing the first glimpses of Australis on the desktop soon enough.