I updated to Thunderbird 17 on Windows 7 yesterday and today Thunderbird is suddenly shown in a mode without any title bar, maximize, minimize or other buttons:
I assume this is the new Windows 8 metro full-screen style.
How do I disable this mode? I have already tried the F-keys and the View menu is also not helpful.
I found the reason: By an unknown reason Thunderbird had disabled all installed themes – even the default one.
When the update was performed the Default Theme was active but two older incompatible themes were still installed. Deleting the two old themes and activating the default theme brought me back the minimize/maximize/close buttons.