Seems like enigmail breaks the rendering of HTML-composed messages. More over, enigmail extension is not working correctly (does not 'unfold' the verified/decrypted messages). Steps to Reproduce: 1) Install the latest seamonkey-1.0.5. 2) Open any HTML-based message the mail reader. Make sure it renderes ok. 3) Open any signed message. Enigmail stars the first-time customization master. 4) Open the message in step 2). It is not rendered as HTML anymore.
Прошу проверить работу с новой версией seamonkey.
Проблема устранена.