For Web developers, Firefox 4 will expand and improve the browser's support for HTML5 technologies, including CSS3 for richer styling options as well as SMIL and CSS Transitions for animation. The new version will also include more powerful developer tools, used for debugging and analyzing pages.
Both the end-user and developer communities will benefit from underlying security and performance work. Just as Microsoft is doing with Internet Explorer 9, Firefox 4 will offer hardware-accelerated Direct2D rendering. This is already showing substantial performance benefits in Microsoft's browser, and is likely to yield a healthy performance boost to Firefox.
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