Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
Windows Consumer Preview added the ability to have XAML / DirectX interop. This had many benefits but is no longer being supported by Microsoft for metro style enabled desktop browsers.
This bug is to go back to using CoreWindow.