Australis is a project to provide Firefox users an updated visual design, streamlined tab strip, new menu and improved customization capabilities. Mozilla is landing the new 'Australis' UI to the Nightly builds of Firefox today.
"Nightly 28 will have Australis, but after uplift (on December 9th) Aurora 28 will not. Nightly 29 will continue to have Australis. It's possible Australis will ride the 29 train through to release with Firefox 29 (April 15th). But we'll be evaluating at each uplift if Australis is ready to ship -- and hold it back again if not."
Download: Firefox Nightly Builds