Contacts Sidebar is a great extension; without it Thunderbird's address book is almost unusable. I tried to contact the original author of the extension many times about reviving the project but did not get a response. The original author's site can be found at the link below. There is lots of good information here about the extension, I recommend taking a look at it.
The code Jeroen Peters wrote is subject to the Mozilla Public License, the General Public License or the Lesser General Public License (see license block below). It is good news for all of us that Jeroen Peters made the code freely available for modification and reuse. It is my hope to restore Contacts Sidebar to a fully functional Thunderbird extension. All code I contribute to Contacts Sidebar will have the same license block
Jeroen Peters used.
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