Printing messages without full headers
From MozillaZine Knowledge Base
To view messages without the full headers, go to the "View" menu and select "Headers -> Normal". This will display messages with only minimal headers. The viewing mode that you have selected should apply when you print messages as well.
To print a message without any headers at all (only the message body), right-click on the message and select "Edit As New..." (or go to the "Message" menu and select "Edit Message As New..."). The message will open in a new window. If you print it from there, it should come out with no headers at all. If you see HTML tables in the message appear with altered formatting (awkward red borders), don't worry, it will still come out ok on the printed page.
You could also use the PrintingTools extension to print a message without headers. It has many other features that can modify the printing layout.