CookiePie is a free firefox extension which lets you log into multiple gmail accounts in firefox without different login credentials. This could really useful add on if for any reason and you have more than one gmail account and you want to login into both at the same time.

You just need to press a Ctrl+T on your keyboard to instruct firefox to open a new tab and then you can right click in the new tab and select toggle on/off cookiepie you will see a small cookie icon on the tab and you are ready to open a different gmail account.

The video below will help you more to understand how to open multiple gmail in a single session in firefox simultaneously

There is another method which involves another firefox extension called IE tab which also lets you open another gmail account in a separate IE Tab.