Why? It is a short and simple question, but I find these sort demands to set unusual defaults like line length are generally misdirected. having lines wrap when the fill the screen space allocated is preferable given you have no idea what screen your recipient may be using to read your missive. It could a a 70nch television of a 3 inch phone screen.

When composing plain text messages, there is a preference in Settings/General/Config. editor, mailnews.wraplength, that defaults to 72 (characters). Setting it to 0 (zero) wraps text to the window width (recommended).

Solution to original problem:

Go to thunderbird settings Under HTML Style

Set FONT - Fixed Width

Remove - Use Paragraph format instead of Body Text By default