The area which has the From, To, Subject section has text entry boxes which line up also with the Formatting bar. The composing area - before it auto drops to a new line if you keep typing is also in same alignment. So if you made the window much wider, that top From, To, Subject section expands and so does the composing area, so text can show more before going to new line. However, this issue is all linked to the 'Formatting Bar' which if enabled needs a given width. So there is in theory, a minimum width that is expected. If formatting Bar is enabled and you can see all of it then there should not be a problem.

But it sounds like you have a serious width restriction on a tiny screen. There is no such thing as a standard screen and computers offer different screen resolutions and scale. MAC's tend to have a smaller pixel size. Laptops do vary in size. I'm on a desktop with separate monitor size 27 inch. So i've got no problems, I can have two Write windows next to each other and still see daylight around the edges :) What size are you talking about ?

Here is my suggestion for a work around - please try it. If you are stuck with the width of the window, then I suggest you hide the 'Formatting bar':

• View > Toolbars > uncheck 'Formatting Bar'

All the Formatting Bar options are still available via the 'Format' menu option of top toolbar. You should now find the text auto wraps as you type.

If you're composing in html format, TB should automatically wrap text to the Write window width. For plain text messages, set mailnews.wraplength to 0 (zero) in Settings/General/Config. editor to make composed text wrap to the window width.