Windows XP only: Freeware app CSSVista offers side-by-side live previews of CSS (cascading style sheets) code in Internet Explorer 6 and Firefox simultaneously.
CSSVista is a three-paned editor—one window is a CSS editor, one window the IE 6 preview, and the third is the Firefox preview. Although Microsoft released Internet Explorer 7 ages ago, boatloads of users are stuck with IE 6. Unfortunately, the rendering engine for IE 6 is an odd bird requiring lots of special attention from developers—especially when it comes to creating cross-browser compatible websites.
CSSVista requires the .NET 2.0 runtime and, despite the name, is a free download for Windows XP only.