Rated 5 out of 5 stars
I finally figured out how to define favourite folders on OS X. In the Preferences of Send and File or when the Send and File dialog is being displayed it's possible to edit favourite folders via the Thunderbird Menu Bar.
Rated 4 out of 5 stars
I really appreciate this Add-On for filing outgoing mails. On OS X (OS X 10.9.5, Thunderbird 38.2) the option for defining favourite folders is missing in the preference pane. Please fix this and I will donate :-)