Firefox 4 by default displays the tabs above the address bar as shown below.
But it also provides option to change the position of the tab to display the below the address bar. This can be done using the View Toolbars menu option.
Click on View menu and navigate to Toolbars menu bar option. From the list of available menus, un tick the menu option with label as Tabs on Top.
Now the Tabs will be displayed below the address as shown below