In Thunderbird, Unless you change the settings, when you forward a message with Thunderbird it will include the forwarded message as an attachment. generally we prefer the text to appear in side the email text body, But how is it possible. so follow below steps to fix it.
This is easy to change.
1. Click on the Tools menu
2. Select Options
3. Select Composition
4. Select the General tab
5. At the top change Forward messages: “As Attachment” to “Inline”
6. Click OK
including text in forwarded messages
Your forwarded messages should now include the text of the forwarded message in the body of the email just below any message text that you send thru Mozilla Thunderbird.