This document shows how to configure Thunderbird to send documents in the plain text format. This could be helpful if you need to send email to an individual that cannot receive emails in the HTML format. These directions work for all current versions of Thunderbird.
Steps to configure Thunderbird to send in plain text:
Start Thunderbird, and click on the Tools menu, and then on Account Settings.
Locate your account on the left, and make sure its options are expanded. If they are not, click the plus sign next to your account to expand the options
Click on the Composition & Addressing button under your account. You should see something like the image below:
Locate Compose messages in HTML format on the right side of the window. Remove the check mark from the box next to this option and then click OK in the bottom right corner to confirm the change. Thunderbird should send all outgoing messages in plain text.