Close, minimise and resize buttons are inactive on main window

My mail client is the Beta release 41. I have just upgraded the op sys to Windows release 10 from release 7. The Close, minimise and resize buttons are inactive on the main Thunderbird window when running on Windows release 10. These buttons are active on other windows such the address book and new message. The keys Ctrl W do work on the main window as does Alt then Exit. Is there some reason that the buttons are inactive on the main window?