I've sort of slipped into using Firefox as my browser. Before that I was a Netscape worshipper. I've just seen that Firefox has passed 5 million downloads. If you're using IE then you should consider Firefox (not least because IE is woefully insecure (even with Windows Update fixes) yet Firefox isn't). Tabbed browsing - once tried, you'll wonder how you coped without.
Out of the box, Firefox is pretty basic, but allows you to easily add on the functionality you want. If you don't know what you want, I would recommend getting All-In-One Gestures and Basics. Cookie Culler and AdBlock are very useful, you might also like Image Zoom. There are plenty of others to take a look at. They're all really easy to install (and remove) too.