Mozilla is on track to debut the new "Australis" user interface (UI) for Firefox next month after promoting the UI to its beta build last week.

Australis, the first visual overhaul of Firefox since March 2011, was added to the "Beta" channel of Firefox 29 on March 20. Mozilla uses a three-stage development process that ships Aurora, Beta and Release versions of each version of Firefox, with six weeks separating Aurora and Beta, another six between Beta and Release.

If everything goes as planned, Mozilla will offer Australis to the release version -- the build most users run -- on April 29. But it's possible that the open-source developer will push Australis further into the future.

Australis 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, but recently shelved its "Metro" version for Windows 8.1.

Many of Australis' visual changes are subtle, with more rounded tabs, inactive tabs that fade deeper into the background and a revamped customization panel. Australis also dumps the orange-colored Firefox menu in Windows, an element that first popped up in Firefox 4.

Mozilla has been working on Australis for almost two years.

Although Mozilla said that "most users are very happy with the new design," citing a survey of those who used Australis on the Aurora channel, it acknowledged criticism as well.

"Of course, as with any interface change, not everyone is happy," the company noted in a March 18 blog about the survey. "With a change this significant, there are some users who didn't like Australis for a variety of reasons."