Most of my browsing is done using Firefox. Apart from the obvious advantage that it is not a Microsoft product, I very much like the ability to easily add plugins. I also like the tabs (although, of course, Explorer 7 has tabs.)
One thing I really like about Explorer 7 though, is the way you can zoom in and out of Web sites. Press CTRL – and the site gets smaller (all to scale), press CTRL + and you zoom in. Do the same thing with Firefox and the text starts changing size, something that eventually screws up even the most beautifully designed site.
The main reason I like this capability is because it means I can include a greater vertical size of Web sites I am trying to capture for inclusion in written materials or as an image on a Web site. Perhaps there is a Firefox plugin that creates a similar behavior, but for now its easy to fire up Explorer.