Mozilla has been working on Australis for more than a year, but it's only now getting to the point where it can show off what it's been working on.
Development of the Australis UI revamp has been moved to the UX Nightly branch, where user experience experiments are usually run.
This means you can now grab a relatively usable build of Firefox with the next-generation UI. Don't expect any polish though, there's still plenty to do before Australis is ready for more mainstream testing.
The basics are there, the rounded tabs, the new menu button, the customization menu, and so on. But it's obvious at first glance that things aren't ready. The rounded tabs look rather jagged, and there are icons missing from the menu.
The polish level depends on the operating system you're using too, as Windows and Mac fare better than Linux.
What's more, not all customization options are yet implemented, and some of the current possibilities won't even be supported out of the box.
However, customization is actually one of the main goals of the Australis revamp and the new customization interface should be a lot easier to use for regular people, most of whom don't even know they can customize their browser.