There are many add-ons available to improve the appearance of Firefox 4. Stratiform is one of the best add-on to tweak the UI in Firefox 4. Here is a new add-on that tweaks the address bar with the Tab.
Flow is a new add-on that combine the tabs and url bar for a clean, minimal, and stylish look.
Flow 1.1 is still in development and it only supported on Windows using the default Strata theme.