Mozilla last week said that slower-than-expected progress on Firefox for Microsoft's Windows 8 "Modern" user interface (UI) means that the touch-based browser likely won't launch until late January.

Estimates a month ago by the open-source developer had pegged Dec. 10 as the target release for the "Metro-ized" version of Firefox. Metro was the name Microsoft once applied to the Windows 8 and Windows RT radical UI, but the company ditched the moniker in August 2012 over a trademark dispute with a German retailer.

The Firefox "Preview Release" will still be packaged with the Aurora build of Firefox 26 for the Windows desktop when it launches Tuesday, as originally planned.

"The goal of the Metro Preview Release is to gather feedback and defects from the larger Aurora community," Mozilla said in notes published on its website from a planning meeting last Wednesday. "Whether or not the Metro Preview Release will graduate from Aurora to Beta and Release channels is still to be determined. At a minimum, it should significantly increase the number of testers and feedback."

Mozilla maintains three public build channels simultaneously: Aurora is the least polished and a precursor to Beta, which in turns leads to Release.

Aurora is to graduate Tuesday to version 26, when Beta and Release shift to versions 25 and 24, respectively.