A common complaint about the current Firefox 4 theme is the wasted space in the title bar, between the Firefox menu and the window control buttons (maximize, minimize, close).
Mozilla is planning to fix this in the next betas, and as usual, it is already providing experimental builds that show the tabs at the very top, at least when the window is maximized. In other sizes, tabs are shown just below the title bar.
The experimental build also improves in discoverability with automatic display of submenus for Firefox menu items.
If you want to try these experimental builds, check Bill Gianopoulos’ web site.