CookieSwap is an Addons for Firefox Browser that allows you to easily manage all of your cookies. So you can do multiple logins to e-mail accounts like Gmail, Yahoo!, Hotmailor anything in the web that you want to login to as different user atthe same time by using a browser that is Firefox + CookieSwap Addons

Here is the download details
Development Homepage : http://cookieswap.mozdev.org/
Download : https://addons.mozilla.org/en-US/firefox/addon/3255

Thanks...