Choose Tools Then Options Then Advanced as follows:
Then Choose "Config Editor" and type in the word "forward" at the top as follows:
Double Click on the first line that reads "mail.forward_message_mode" to yield the following screen:
Change the value 0 to 2 then click on OK.
Congratulations !! You have now changed the way Thunderbird is configured to handle forwarded messages. You can always go back into this same screen and change the value back to 0 in order to make it send forwarded emails as attachments.