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You need to tell Thunderbird how the original message should appear in the Forward and or Reply window. To do this, open Thunderbird then:
For the Forward
Click Tools > Options > Composition button > General tab, then from the drop down list next to "Forward message" select Inline.
For the Reply:
Click Tools > Account Settings, then click on "Composition & Addressing" in the left pane, then check the box "Automatically quote the original message when replying"
Hope this helps. Let me know if you have any questions.