Mozilla has released mock-up themes for the new Firefox 4.0. They seem to be mulling over whether to have the tabs on top, a bit chromish or to have the tabs at the bottom. They have released both the themes probably to get an idea of the response.
Mozilla is thinking aloud on the pros and cons of locating the tabs. The advantages of having the tabs on top Mozilla feels are, Vertical space will be saved, extra visual element will be added as the tabs have to connected to some thing. Distance to page controls will be shortened.
The disadvantage of such a move Mozilla feels are that the users will be confused by the unfamiliar location. As space is removed from the title bar only shortened version will be displayed in the tab. Some space may be lost as the tabs will be sandwiched between the application icon and the window widgets.

The other change includes adding a book mark widget in place of a Book Marks menu or tool bar.
It is also intended to attach a combo/Refresh/ Stop/Go button that is at the end of the location bar. When at rest the button does not change its color and is the same as the location bar. When some thing is being typed in it turns green in response and when a page is being loaded it turns red.