Starting with the Mozilla Thunderbird 17, the classic title bar has been hidden in Thunderbird. To get the classic title bar back, follow these steps:
- Start Mozilla Thunderbird.
- Open the config editor by clicking on Tools -> Options -> Advanced -> General -> Config Editor (button)
- Navigate to the setting "mail.tabs.drawInTitlebar" in the config editor and set it to "false" by double clicking on the line.
- Close the config editor and restart Thunderbird