Presumably you can still use your computer - or did you post this from a different one? Can you scroll/move this email to get to the x (close) button. Or even the size button. Or can you restart or otherwise close TB (ctrl+alt+del and use task manager if needs be)? And then restart TB and check to see if any other emails do that same thing? i.e. is it a TB problem or just that one Email?

Hi, yes I can still use my PC. However, I use two screens and the Thunderbird screen was open on the larger of the two, but when I moved it to the smaller screen, everything displayed OK. So am opening it on my smaller screen for the time being. Any thoughts?