When messages are forwarded from Thunderbird, by default, they are sent as an attachment to the new message. If, like me, you prefer that the forwarded message be sent inline (as in readable at the bottom of your new message), then you’ll need to change the default setting. Here’s how:
On a Mac, go to Thunderbird > Preferences; on a PC, go to Tools > Options.
Go to the Composition tab, the General sub-tab.
The first option at the top deals with how messages are forwarded. Switch that setting from “As Attachment” to “Inline”