Rated 4 out of 5 stars

I used the Contacts Sidebar of old and was disappointed when it stopped working with newer versions of Thunderbird. Hence I was pleased when I came across Custom Address Sidebar.,

While it is not as conveniently located (you have to open the 'Write' panel rather than view it in the main Inbox view of Thunderbird) it does offer a very useful search facility, which can be configured to readily find a contact from your address list. - Be aware that you may need to enable the sidebar from the View menu, though pressing F9 also toggles the sidebar on or off.
Initially I was expecting the sidebar to appear in my main Inbox view which, as mentioned it does not, and the supplied images in the Add-Ons page didn't provide enough information for this poor English speaker to work out how/where to use or access it.
Remember Inbox> Write> View> select Contacts Sidebar to see it
See the response below for a link to an English translation of the developer's instructions.

This review is for a previous version of the add-on (1.1.1-let-fixed). 

Please visit this site, you can know how to use this add on.