Instead of having tabs at the top of the screen, Metro-style Firefox may use a sidebar, with thumbnail images for each tab.
Next month's Firefox 12 will be the last version of Mozilla's browser to run on early editions of Windows XP and the 12-year-old Windows 2000.
At some point during March, Google lifted the penalty it had imposed on Chrome the first week of January, when it demoted the search ranking of the browser's download page.
Mozilla is currently testing default encrypted Google searches for all Firefox users, with the intent to make all Google searches encrypted in the near future.
Mozilla hailed features added this year such as add-on compatibility and synchronization improvements, silent updates and better organization of development tools.
Mozilla's new browser will be available for manual downloads later today, but automatic updates will wait until the effect of Microsoft's Windows fixes are better understood.
Due for official release on Tuesday, this latest browser version offers faster page loads, add-on synchronization, and smooth migration from Google's Chrome.
Developers at the Mozilla Foundation say a Windows 8 version of its Firefox browser is in development and will be ready for beta tester by the summer.
As Chrome's market share takes a hit from external factors, Firefox continues to hold its own in the No. 2 spot, new data suggests.