First open Firfox and then login to Orkut.
After logging in Open a new Tab in Firefox and click on close button of the browser
You will get a Dialogue box Click on SAVE
Then again open Firefox, you can see you are still logged in
Then On your Orkut home page, scroll down and at the bottom you will see go to orkut.com
Now you can see a new login screen has come
Login with your second account.