Although only about 5% of internet users use Firefox, there is reason for IE to be nervous. First, Firefox is new. Its email client Thunderbird has only been officially available to download for about a month. And Firefox doesn't really advertise. Everyone I know heard about it from a friend, from a blog, or from a online article saying its much more secure than IE. But most importantly, it's use is growing fast, almost as fast as everyone's frustration with IE.

Read Internet Explorer Use Still Falling, Firefox on the Rise.