Internet Explorer 7

Most likely you've either downloaded IE7 or heard about it somewhere. specially because of the large number of articles comparing IE7 and Firefox2. Just let me add here some exciting features or behaviors I noticed about IE7. Whenever you log in or log out of a Google service in one tab, it automatically does the same thing for other google services open in other tabs and refreshes them too.
The other thing is its ability to zoom in or out of a page. I am not sure if Firefox has such an extension but it comes handy sometimes, especially when you want to look at a small picture.
Although I am still a fan of Firefox, I recommend you to explore the new IE7 yourself.