user.js settings I use:
Setting  | 
  Value  | 
  Rationale  | 
font.size.systemFontScale  | 
  100  | 
  Default font scale  | 
mailnews.wraplength  | 
  0  | 
  Disable text wrapping in composer  | 
mail.strictly_mime  | 
  true  | 
  Use base64/quoted-printable only, do not use 8bit/7bit encoding. Addresses a known problem with Zoho.com.  | 
Unknown settings
I had these set at one point or another, but I don't quite understand what they do.
mail.compose.default_to_paragraph
Composer window line-height
In omni.ja:chrome/classic/skin/classic/messenger/messageQuotes.css: