Life is a complicated proposition, and your online life is no exception. You probably have several different email addresses you use, and lots of different logins for various sites you frequent. But if you have a couple of different Gmail accounts, for example, typically you have to log out of one before you can log in to another. Sometimes it might be helpful to be able to log into both accounts at the same time.
Multifox is a Firefox add-on that allows you to assume multiple identities at once. Instead of having to log out to change from one Gmail account to another, for example, all you need to do is tell Multifox to add a New Identify Profile and you're off to the races. This tool keeps separate login information, lists of open tabs and windows, and individual sets of sets of cookies for each profile. You can open a window from the menu as a new profile or right-click on a link and follow that link as that other identity.
Multifox is available for both versions 3.6 and 4.0 of the Firefox web browser.