Workaround is to hit more > view message using standard reader.
The problem is twofold.
aUrl is a nsIMsgMailNewsUrl which seems to have a mimeHeaders property http://mxr.mozilla.org/comm-central/source/mailnews/base/public/nsIMsgMailNewsUrl.idl#97 which we could pass to the nsIMsgMdnGenerator...
The main issue is that this is a separate header (the one that tells that a delivery notification has been request) that gloda messages don't know about, so we would need to write a gloda plugin for that as well...
Workaround (2y old) is useless since I don't want to send read confirmation (bad habit I know, but sometimes useful) and it's send automatically when I view the message in Conversation (which is immediately in Message Pane)... Is there another way around?
Fixing that problem to this day still requires significant rework on the Thunderbird side, so unfortunately there is no other way around...