Firefox 4 to have tabs on top
As you may know, Firefox 4 will have the tab bar placed on top of the navigation bar by default, significantly changing the way it looks and, to some degree, behaves for long time Firefox users.
As Alex Faaborg emphasizes in this video, this is all about the default layout. Reversing to tabs below the navigation bar is a matter of right clicking on the toolbars and unchecking this option.
Source: Mozilla Links