Wrapped text is generally the default, although I sometimes see otherwise. Thunderbird retains size of message window and opens subsequent messages in same size window as prior message. That is, it does not increase or decrease based on line length of message. That has been my experience, although I have not seen it in print anywhere.

Thanks David for your post When I resize the message window, it still doesn't wrap the text until I go back to full size. But yes the window width is maintained as previously set. However, this appears to only apply to some non commercial emails, that is normal text from a person or small company, whereas most emails coming from established companies appear to set their emails to a specific width which fits moderate width message windows, and still stay this size if the window is full size. Geoff