Tente este addon do Word Wrap
Unchecking"Enable Word Wrap" in the Options menu (or pressing the keyboard shortcut) will temporarily turn off word wrapping for the current message - lines will take up the whole width of the composer window, and no line breaks will get added to the outgoing message (with the exception of "hard" line breaks inserted manually).
Checking "Enable Word Wrap" will enable word wrapping again and set it to the standard width (which is modifiable through the "mailnews.wraplength" preference in the Config Editor, the default is 72 characters).