I find that when I paste excerpts from my text editor (width 100) into the thunderbird compose window it is truncated to width 80 or so by the time it gets to the receiver. Within the compose window itself, there is no truncation. I can verify this by sending the Email to myself. I am not sure if these extra end of line characters/formats are inserted before sending or on receiving. Is there a way to tweak this behaviour so that the width of my Emails are always 100?
You can change the number of characters per line by changing the prefs value mailnews.wraplength. By default it contains the value 72. Change it to 100 and plain E-Mail messages in the composer will be wrapped at column 100.
The Config editor can be opened via: Tools → Options → Advanced → General → Config Editor