4 years ago was the last update of this extension which was then abandoned by its (now unknown) author and eventually removed from AMO. I used it ever since when recently it started glitching a bit, so I decided to revive it with a small improvement upon the original: you can close it the same way it opens - with a click on its icon in the toolbar. The extension has finally been approved by AMO's team and it's accessible to be downloaded and installed. Screenshots and a video demo were added in the description on AMO.

https://addons.mozilla.org/en-US/firefox/addon/downloads-sidebar-reborn/