It has only taken Mozilla 8 (well, no 7 as there was no Thunderbird 5) Thunderbird releases to catch up with the UI change made in Firefox 4. The Daily builds of Thunderbird 12 now have the tab bar at the top of the window. Presently though, there is not an option not to have the tabs on top. Thunderbird 12 is schedule for release on April 24, 2012.