Some people have more than one Facebook account. They use this technique to keep their personal and professional life different.
For example, businessman will have his personal id and professional id different. His professional id will be used for business and product promotional purposes. In this case, people want to know what is going on with his friends in their personal account id whereas simultaneously check what is the status of his product using Facebook fan page.
In Firefox, this is not possible, but people using Google Chrome browser have this facility to use many accounts at the same time.
One tab will be your personal account and the second tab will show your second account.
When you want to do so, just open Google Chrome browser. Log in to your first account in a tab, then click on the "Settings" option from Chrome browser. Another tab will open, you need to click on "Users" option and type the user name of second id in the text box provided. Then click on "Add New User" option.
In this way you can log in to as many accounts as possible in Chrome browser.