from: http://kb.mozillazine.org/Filters_(Thunderbird)
By default, Thunderbird doesn't support filtering the message body in a IMAP account. There is no "Body" in the list box. However, most versions of Thunderbird will let you filter the message body in a IMAP account if you add "Body" as a custom header . Either create a new filter or edit a existing filter and select "customize" in the left most list box. Enter "Body" as the "new message header". This adds user_pref("mailnews.customHeaders", "Body");' to prefs.js. If you have multiple IMAP accounts you only need to add the custom header once, any custom headers affect all accounts.
Unfortunately, the support for custom headers is version dependent. In version 1.5.0.7 you need to use "Body:" (add a colon to the custom header), many older versions accept both formats, while in 1.5.0.8 it complains about a invalid character if you enter "Body:" in the GUI but has no problem using "Body:" if you added it using a text editor. You need to experiment.
Chris
