Australis is the project that represents the latest user-interface changes for Mozilla Firefox across Linux, OS X, and Windows systems. There is still work to be done on Australis, but it's making beautiful progress.
According to Mozilla, Australis is the "most beautiful and detail-obsessed iteration of Firefox’s visual design yet: modern, clean, and comfortable." The Australis project simplifies and streamlines the default UI for Firefox but still offers a wealth of customization opportunities.
For those looking towards a ton more information on the Australis interface that's available in the latest Linux nightly builds of Firefox, see this excellent Mozilla.org blog post.