Last October, we launched a rewritten code base for the Gmail user interface to Internet Explorer 7 and Firefox 2 users. Since then, we've added support for Safari 3 and Firefox 3 and improved performance in other browsers. This new code base included major performance improvements and provided us with a solid foundation for launching new features such as colored labels, group chat and rich emoticons, invisible mode, AIM integration, Gmail Labs, an updated contact manager, and remote sign out.
This week, we've started to roll out Gmail's new code base to IE6 users. If you use IE6 and have the latest IE6 updates from Microsoft installed (or the specific patch that's required), you'll start seeing the features listed above.
Link - from Gmail Blog
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