Is it possible to send me the source code of that e-mail? (Press Ctrl+U in Thunderbird) You can attach it here. Also, what version of Thunderbird are you using? Is it an e-mail generated by rc1 or an existing e-mail?
I am using Thunderbird 3.0.4 and I see the e-mail fine. Initially I see only the text and after clicking on "show images" (or something similar) it loads the CSS which causes it to display images instead of the text.