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. Bye, Mic
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.