This is accomplished via the New Session option, which makes it possible to log onto Web sites that track your identity across different tabs--like Gmail.
In Firefox, for instance, you can't have multiple Gmail accounts open in multiple tabs or Windows. If you sign into a second one, you'll get signed out of the first one when you try to do anything.
Here's how to take advantage of this IE8 perk:
1. Run Internet Explorer 8 and open Gmail. Make sure the Remember Me option is unchecked when you sign in.
2. Press Alt-F, I, and then hit Enter. This will open a new Internet Explorer session (which for all intents and purposes is the same as a new IE window).