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Click Tools > Options > Composition tab, then select 'Inline' from the drop down box next to Forward message..
Hope this helps.
Ok, on the mac I think you will find it under Preferences > Composition > General, then look for 'Forward Messages' and select Inline from that pull-down list.
Now test it by opening any received message in your inbox then click the Forward button, type some text in the message box, then address it to your own email address. When the email gets received in your inbox you will see the message that you typed earlier appearing in the message box rather than as an attachment.
Ok, try to compact your folders (File > Compact Folders). If problem persists then you probably have some corruption in TB program files or registry in which case you will need to reinstall Thunderbird.