A switch for Firefox's native sidebar (bookmarks, history, etc). It's a slim vertical button on left of main content.
Replace the legacy addon All in One Sidebar. You can open sidebar by clicking edge of screen.
This is a Firefox userChrome JS script.
Tested on Firefox 91.
This repo only contains the specific function, doesn't contain the code to enable userchrome scripts.
For how to enable, see: (also more of my scripts)