user.js settings I use:

Setting

Value

Rationale

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.