Good idea, but... Rated 3 out of 5 stars
Works exactly as says on W7. However, on OSX it does nothing at all. Please specify that is a Windows-only add-on. Thanks.
(Update 15 May 2014)
Thanks for your quick reply. The problem with OSX is probably due to the fact that Thunderbird notifications are channelled through Growl. Keep up the good work,
(Update 22 May 2014)
Growl notifications remain active even with the "make draft unread" option checked. You will need to test it again on OSX using Growl.
Not only for Windows
this addon is not meant to work only on Windows. I developed it on Linux and tested on Windows and Linux and it works.
Sadly I had no way to test it on OSX, but I'm not using any system specific function. I'll find an OSX system to test it on it and verify what's happening.
UPDATE 16 May 2014
I tested it on OSX, without Growl, because is not a free app, and googled the problem. It seems all depends on how Thunderbird manages new and unread mails on OSX: it doesn't differentiate them as on linux and win.
So the only way to use this addon is to choose the "make draft unread" option.