Review for Contacts Sidebar by Stephane
Rated 5 out of 5 stars
There is a solution to use it after Thunderbird 52:
follow this http://www.mozbrowser.nl/forum/viewtopic.php?t=23983
or download it here: http://www.meteo.be/resources/misc/contactSideBar/contactssidebar-1.7.1.zip
To create your own collections, you must have a Mozilla Add-ons account.