When I am composing an e-mail message in Thunderbird, the individual lines of text occupy about half the screen width, but when I receive messages, even ones I've composed and sent to myself when checking layout, they display on the full width of the screen. In many cases this isn't a problem, but if I do some cutting and pasting, importing text from elsewhere, or re-wrapping when composing a message, it can end up after it's been sent with some sentences or sections occupying half the screen width and others the full width, which must be irritating for the recipient in trying to read or print it.
How do I set the line-length when composing so that under any circumstance it will display over the full width of the screen, so that what I type, irrespective of cuts, pastes, re-wraps etc, appear the same as what I or others receive. I've gone through the tools-options menu and the tools-account setting, but can see no reference to setting line length for message composition.