The Australis interface is only available in the nightly build, but it's a good start
The first Alpha update for the new Firefox 28.x branch is now available for download and testing on all platforms – Linux, Windows, and Mac OS.The binaries for Mozilla Firefox 28 Alpha 1 can be downloaded and run without having to install anything. This is especially helpful if you don't want to install software deemed unstable and you don't want to mess up the stable version.
Mozilla doesn't usually publish details about intermediary Alpha releases, and this one makes no exception. They sometimes offer more info about Aurora builds, but in this case it's just a download link.
The best feature for this particular version is the Australis interface that has been pushed in this build. Resistance is expected from the community, but you better get used to it, because it's going to happen anyway.
Download Mozilla Firefox 28 Alpha 1 for Windows
Download Mozilla Firefox 28 Alpha 1 for Linux
Download Mozilla Firefox 28 Alpha 1 for Mac OS X
Remember that this is a development version and it should NOT be installed on production machines. It is intended for testing purposes only.