jedakay,
I am sorry that I misunderstood your original problem description.
Yahoo offers its users a choice in how to create mail messages.
1)create all messages in plain text with the default Yahoo chosen font
2)turn on 'color and graphics' that allows choice of font, colors, emoticons, text formatting etc.
No surprise that many users opt for choice 2.
Users who use the default choice (1) have messages sent out that avast has no trouble adding the "clean" message to and it is always displayed in these Yahoo messages when received in Thunderbird.
When a Yahoo user selects option (2) and creates a straightforward message then Yahoo sends the message out containing two different versions of the message. This is to allow for someone who might still be using an old mail client that cannot deal with the rich text formatting or users who simply choose in their mail client to view plain text only.
Yahoo sends these message as 'multipart/alternative'. That simply means that the message contains two body parts and they are alternatives for the mail client to display. One part is the message in plain text - and avast can easily add the 'clean' message to that part. In Thunderbird you can see this by displaying the message then go to "View" > Original message body as > Plain text.
The default in Thunderbird is to display the message in the rich text format which is really the message turned into a web page so that all the formatting, colors, fonts emoticons etc can be rendered. It would be very complex for avast to add the 'clean' message to this web page. It would need to completely decode the html and then rebuild it to add the 'clean' message in a way that did not disrupt the way the message was intended by the sender.
Because Yahoo creates the message as multipart/alternative it means there are just two body parts - avast cannot add another body part without completely rebuilding the mail message from scratch.
You may notice that, if your Yahoo correspondent sends you a message with an attachment (such as a picture), then Yahoo has to create a more complex message that is not limited to two body parts. In this case avast is able to add an extra body part containing the 'clean' text and it is displayed by Thunderbird at the end of the other message parts.