There are a couple useful extensions that will undo the UI changes introduced in Thunderbird 17 as well as the changes to the tabs back with Thunderbird 11. Both of these will work together fine, or you can install on or the other depending on how you would like your Thunderbird UI to be setup.

• Rise of the Tools – Moves the tabs below the menu bar. Thunderbird still does not have a ‘Tabs on Top’ option that can be changed via the View Menu like that in Firefox. No Restart Needed
• Title Bar Tabs Cleaner – Restores the Windows Title Bar without having to jump through a (as many) hoops to disable this option via the preference editor. However, once installed and after restart you still need to add the Titlebar Cleaner button on to your menu bar and the click the button to toggle the title bar on/off.

There may be some quirks with the Title Bar Tabs Cleaner add-on. On my main profile, the button has disappeared as soon as I had clicked it to restore the title bar. On a different profile though, the button remained. You can still manually ‘turn on/turn off’ the title bar via the config editor tweak mentioned in this post (which I think is actually quicker as you don’t have to install the add-on, restart Thunderbird, go into the Customize, add the button, exit customize and then click the button).