How to login to multiple accounts at the same time

Do you have more than one accounts of the same website? Google, Facebook, Yahoo or Twitter perhaps? If you do then you must’ve realized that you can’t log in to your second account without logging off from the first one. In other words you can only access one account at any one time. Frustrating isn’t it? But hey if you are a Mozilla Firefox browser user there is good news: An add-on called