One of the most prominent changes in Firefox 4 is the default position of the tabs which is moved to the top area above the address bar. This position change is supposedly to make the tabs more visible and more convenient to be managed. However, Firefox 4 also provides an option to move the tabs back to their old position, that is below the address bar.
To move Firefox 4 tab position from top to below address bar, click the Firefox small button. Select “Customize” from the drop-down menu and remove the check mark on “Tabs on Top” by clicking it. Once you click it, the tab position is moved instantly to below address bar.
The final version of Firefox 4 has removed the Customize menu from the Firefox button. In fact, there are a lot of changes over there. The option “Tabs on Top” is now moved to the “Options” menu (as you can see from the first screenshot above). Remove the check mark by clicking on it to move the Firefox tab bar from top to below address bar.