ZD Net's Mary Jo Foley heard rumors that Microsoft is developing a new browser that will make its debut with the introduction of Windows 10. This new browser is not IE12, it's a light-weight browser codenamed "Spartan". In terms of look and feel Spartan promises to be more like Chrome and Firefox.
Spartan is just a codename at this point, the final name for this product hasn't been determined yet. Foley writes some features of the browser may be shown off at Microsoft's Windows 10 features event on January 21 but notes that the browser is unlikely to be functional enough for inclusion in the Windows 10 January Technical Preview, it may not show up in the test builds until some point later.
Thomas Nigro, a Microsoft Student Partner lead and developer of the modern version of VLC, mentioned on Twitter earlier this month that he heard Microsoft was building a brand-new browser. Nigro said he heard talk of this during a December episode of the LiveTile podcast.
However, if my sources are right, Spartan is not IE 12. Instead, Spartan is a new, light-weight browser Microsoft is building.