Mozilla plans to release the Windows 10 version of its Firefox browser in August or September, and is already showing what the new browser will look like.\nThe open-source developer revealed earlier this month that it had reversed course and would produce a browser for Microsoft's new operating system, which arrives on July 29. Mozilla had abandoned that idea early last year.\nPrompting the change of heart: Windows 10's express setup changes previous browser defaults to Microsoft's own Edge during an upgrade, according to published documents. That, according to Mozilla officials, could pose problems for Firefox, which has seen its market share cut in half over the last couple of years. It now accounts for about 12% of browsers in use.\n"More than 70% of our user base will be eligible for the free Windows 10 upgrade on desktop," Mozilla said. "After the upgrade occurs, users will be exposed to messaging encouraging them to switch to the Edge browser. This poses a retention threat for Firefox. ...Upgrading to Windows 10 will create change\/friction. We should leverage that moment to put Firefox top of mind."\nHere's what Mozilla envisions:\n\nLaunching the new browser shortly after Windows 10 arrives. According to publicly posted information, Mozilla hopes to support Windows 10 with either Firefox 40 or 41. Firefox 40 is set to ship on Aug. 11, Firefox 41, on Sept. 21. The best bet is that Firefox 40, which will have major design changes and updates, will be ready for Windows 10.\nThe developer earlier this month published design specifications for the Firefox UI on Windows 10. That look is similar to current Firefox editions on other platforms, including Windows 8 and OS X, and, like Edge, will offer "light" and "dark" themes to match the operating system's options.\nMozilla's main goal -- "engagement" -- will be to "retain Firefox users who upgrade to Windows 10," while also vying to "acquire IE and Edge users." In another planning piece posted by Mozilla, the developer outlined the messages it will deliver to users, and thus the reasons behind the Windows 10 project itself.\n\n"We don't know exactly what the [Windows 10] update trajectory will look like," Mozilla said in a document. "Our goal is that when Windows 10 releases start to go out, Firefox will work well on Windows 10."\nWith reports by Gregg Keizer at Computerworld.