Bug 1023511 and bug 1023517 cover a lot of HiDPI theme work that either already exists in the OS X theme (for Retina), or soon will. But there are other changes that will be needed on Windows, for things that don't exist or don't apply to other platforms. From a survey of our current UI on Windows, this roughly includes: - Page Info section icons - Full-screen mode window widgets (extreme upper-right) - Application icon in the taskbar - Image assets in the installers - large panel images in the stub installer - regular installer left-side image and icon. A few things that may be platform bugs, or that I didn't see specifically covered in the Retina work: - Twisty arrows in <tree> expanders (eg in bookmarks sidebar) - The arrow in menuitems with a submenu is hidpi but really tiny. But I also saw this in some native Windows apps. Not sure if this is expected, or just a bug with Windows 7's incomplete HiDPI support? Finally, we should probably have a UX review for any other desired HiDPI theme changes. As a specific example, I noticed that Windows apps tend to frequently use superfine borders (1 physical pixel) in various places (like scrollbars and textfields), whereas most of our Retina UI has retained using 1 CSS pixel (== 2 physical pixels, consistent with other OS X stuff, but not Chrome).

If you need a QA guinea pig I have a HiDPI Windows 8.1 device.