Tbird needs the ability to set a right-margin when inputting text.
Tbird treats all input as one, long, continuous line broken only by screen width or hard returns. But for text- intensive emails that are lengthy and multi-paragraphed, shorter lines are much easier to read. At present, in order to use shorter lines, one is forced to type as though using a typewriter, periodically hitting <Return> each time a chosen line length is reached, that or go back and edit the line lengths throughout the message.
Recognizing that others' email clients won't necessarily have the same feature, the effect of a user-set right margin in Tbird would be for it to automatically insert a hard-return each time the line limit is reached. If that happens mid-word, then it would back up to the last space, and simply drop that word to the next line. Going back and hyphenating a few words manually thereafter if necessary would be a lot easier than having to through an entire message line-by-line as is now required.
What's more, if something like that could be implemented, a true justified right margin could also become possible.