The latest nightly releases of Firefox.next come with customizable tab bars, which means you can drag buttons off (like the new tab and list all tabs buttons) and into the tab bar to make it suit your needs.
This can be used to reduce all Firefox toolbars to a single one as shown in the screen shot. Right now there is no way to set a minimum size for the location or search bars and they are just compressed as you open more tabs, becoming unusable when working with more than 7 tabs (in a 1280px wide screen).
On Windows, you can also notice the slightly updated tabs style: more separated and rounded and more contrast for the active tab.
Important changes are still to come for tabs including nice animations when adding, closing and moving tabs, and Aero Glass for all toolbars on Windows Vista and 7.