Microsoft is happy with their current market share in browsers and as a result, there has been little or no development in Internet Explorer for several years. From what I have heard, no major upgrade of IE is planned and you might have to upgrade your Windows version to get their new browser when their new version of Windows comes out in 2007(?).
I am currently using Firefox and still use Mozilla from time to time because I also use the composer. I use the tabbed browsing all the time and the built in pop up blocker is a must have. It also has some nice features the average user might not need like a nice cookie manager. You can "fix up" IE with third party products but too many pieces of software makes your computer into a support nightmare. It is much cleaner to install and use a browser that is up to date and has all the features you need without the complication of third party add ons. The fact that it is free at www.mozilla.org makes it that much better.