Firefox has officially announced availability of its new "Australis" theme for testing purposes. The redesigned interface brings in a number of visual and UX changes to the popular open-source browser. For example, browser tabs now sit higher in the titlebar, have a fluid shape (like Chrome tabs) and are visually de-emphasized when in background but slide forward into focus on a mouse rollover.

Other features, some of which had already shipped in earlier versions, include a forward button that shows up only when there's somewhere to go forward to, a download button that converts into progress bar during an active download session, a button to easily fetch saved bookmarks, and more.

While the most frequently used controls are prominently displayed as buttons on the toolbar, there is also a touch friendly menu towards the far right, which lists widely but less frequently-used controls. Australis also features a customization mode through which the menu and the toolbar can easily be tailored to your liking.

This is said to be the next generation of Mozilla's Firefox user experience and the "most beautiful and detail-obsessed" interface yet. It simplifies and streamlines the default interface and makes it more extensible to accommodate future features. It also brings in consistency and unification to the look and feel of the browser across various devices. 

The theme was first previewed in 2012 and according to Mozilla, it still needs more polishing. To download the latest nightly build (Windows, Mac or Linux) please head here.