Unnamed sources have informed ZDNet's Mary Jo Foley that Microsoft plans to launch a brand new lightweight browser. Codenamed "Spartan," this browser is expected to make its debut on Windows 10, and it will feel more like Mozilla's Firefox and Google's Chrome browsers than Internet Explorer. Windows 10 is slated to go retail in 2Q 2015 or 3Q 2015.
As with Microsoft's virtual personal assistant (Cortana) and Windows 10 (Threshold), Microsoft is digging into Halo lore with the Spartan codename. According to the Halo Wikia, the name describes "members of several super-soldier programs instituted by the UNSC."
Sources told Foley that Windows 10 for desktop will ship with both Internet Explorer 11 and the Spartan browser; Spartan will not be Internet Explorer 12. However, Microsoft plans to release a version of Spartan for mobile platforms, as well. What's not clear is whether Spartan will be a Windows 10 exclusive or if it will be made available for all operating systems such as Android and Apple's Mac OS X.
As it stands now, Windows 10 Technical Preview includes Internet Explorer 11.0.9879.0. The next release of the Technical Preview will take place on January 21, 2015, during a special event that will reveal the platform's consumer-focused features. Spartan may be revealed during the event, but currently, there's no indication that the browser will be released at that time.
Based on the provided information, Spartan has a good chance of becoming Microsoft's first multi-platform browser. Firefox and Chrome already address the multi-platform market while Internet Explorer has served as a Windows exclusive. Sources suggest that Internet Explorer will still be offered for backwards compatibility reasons.
Spartan fits in with Microsoft's multi-platform services vision (opens in new tab). The company currently offers Office apps for both Apple's iOS and Google's Android platforms. However, Modern UI apps for Windows 8.1 have yet to be released.