Personally, I dislike HTML email. I think email should be text only and want to use Thunderbird to send out plain text emails. Out of the box, Thunderbird is setup to send HTML formatted emails.
Click Tools -> Account Settings
Click on Composition & Addressing
Uncheck “Compose messages in HTML format”.
Now click Tools->Options
Change “Forward messages:” to “Inline”
That should take care of it. Now you can send out plain text emails.