I have Thunderbird under Ubunbtu in two Lenovo notebook computers. After the automatic update to version 17, the Top Menu disappeared from one computer. This is a quite annoying bug, as Thunderbird is always maximised, as there are no buttons to close or minimize, so Thunderbird completely covers over the menu buttons for the operating system. With every update from version 18 to version 24 of Thunderbird, I hoped this bug would disappear, but it remains. Upgrading the Ubuntu operating system from version 12.04 to 12.10 to 13.10 to 14.10 made absloutely no difference. Thunderbird has no Top Menu and no Close or Minimise button. Some said they stopped this bug by setting mail.tabs.drawInTitlebar to "false". It makes no difference to either computer setting this to either "true" or "false." One computer retains a Top Menu (Title bar" and the other never gets one. I am not using any themes, and I have many times compared EVERY setting between one thunderbird installations and the other, and there is no difference (except one has my email and the other has my wife's email.)