From the dev list:
I'm working on a new button in toolbar (mail.html and message.html
templates). This will be a popup menu like markmenu,
will contains container for plugins and three actions (for start):
- open message in new window
- show source (in new window)
- Download as .eml file
The button will be enabled in message view or in mailbox view when one
message is selected. I'll remove current viewsource button.