Mozilla on Monday began rolling out its first major Firefox user interface (UI) change in more than three years, seeding early adopters of the "Nightly" build with the new "Australis" revamp.

Nightly builds are designed for preliminary testing, and as the name implies, are automatically updated each night to that day's edition. Firefox's Nightly builds are Mozilla's roughest-edged editions.

"[Australis] is the next iteration of the Firefox user interface," said Madhava Enros, the head of Firefox's user experience (UX) design team, in a Monday blog. "It's not quite finished, and it needs more polish, which is exactly why we're so eager to get it out to a wider set of the community."

Mozilla last refreshed the browser's UI with Firefox 4, which launched in March 2011. It has been working on the new UI since at least May 2012, when Enros first brought up the changes.

Australis, said Enros, is Mozilla's attempt to both streamline the browser's UI and standardize it across all platforms, ranging from the desktop -- where versions are available for Windows, OS X and Linux -- to mobile. Mozilla has a version of Firefox for Android and is working on a "Modern," or "Metro," app for Windows 8.1.

Most of the visual changes appear relatively subtle. Tabs, for example, are more rounded than in the current production edition, and inactive tabs fade deeper into the background.