Most of them who are having multiple accounts on Gmail, Yahoo, Orkut, Facebook (or any other online web service) will be knowing that they couldn’t login to more than one account at a time. That means if you are using one Google account for Gmail and other Google account for Orkut then you will have to logout of one to access the other.
CookiePie,a Firefox addon solves this problem for you.CookiePie is an Open source plugin for Firefox/Flock that enables you to maintain different cookies storage in different tabs and windows.
How to Use CookiePie
Download CookiePie from their Official homepage.After installing the extension, restart FireFox.And now for logging into multiple accounts of Google,Yahoo or any other service, just right click the new browser tab and toggle on/off CookiePie.This will give that window a separate environment for its own unique cookies and you could be able to login to the other accounts simultaneously.
Firefox, Flock and GNU IceWeasel.(I checked only with FF 3)
Windows and Linux.(Sorry Mac users,this is not for you :p)
Checkout the Video of CookiePie Working with Gmail!
Related:Firefox Bookmarking Extension