Rated 5 out of 5 stars

I really like your addon.

It would be nice if you add an option(tick box) to message to use system message instead of custom window, or at least add a hint for creating "Execute command" that does this. like:
/usr/bin/notify-send -u normal -i /usr/lib/thunderbird/chrome/icons/default/default16.png "Unread mail: %count" "%sendername : %subject

This review is for a previous version of the add-on (0.22.1).