There are many Internet User who love to have more then two Gmail, Yahoo, Orkut, Facebook (or any other online web service) accounts and they have habit of surfing with all this account but firefox doesnot provide any facility to login to all account at once ,you have to logout your current email account to login another account but all this is possible by using simple tweak inside yours Firefox browser or via using third party tools or plugins.
CookiePie ( Install ) is an open source plugin for Firefox and other browsers that allows the user to open multiple accounts for services such as Gmail, Yahoo, eBay, Facebook, Twitter, or others in different tabs or browser windows simultaneously. It can also be used by developers who want to work with different versions of web pages with each tab or windows having its own distinct cookies environment.
CookieSwap ( Install ) is another plugin that enables you to easily swap all your cookies so that you can be logged in to multiple web e-mail accounts (like Gmail and Yahoo! mail) at the same time and quickly switch between them.You can also use IE tab Firefox extension for login into two google accounts(or any other online services) simultaneously
You can use all above service for Gmail,Microsoft , Hotmail,Yahoo Mail,Facebook, eBay and Twitter account.