In the new Thunderbird 3, Mozilla included tabbed browsing. Since you usually don’t have tabs open all the time, it would be nice to be able to hide the tab-bar when no tabs are open. There is a hidden option to accomplish this. Thanks to Truben for pointing this out! Read his article at http://truben.no/journal/auto-hide-tab-bar-in-mozilla-thunderbird-3/
Hiding the Tab-bar in Thunderbird 3