One of the most popular Firefox add-on — Tree Style Tab has been ported to the really fast Quantum based Firefox releases (57+). For some reason, this plugin is not yet able to auto-hide Firefox native tabs, so users now have two tabs — one at the top and the other at the side.
Until Tree Style Tab adds auto-hide functionality, the temporary solution is to hide the native tabs manually, using userChrome.css file.

First of all, go to about:config, and change the value of toolkit.legacyUserProfileCustomizations.stylesheets to true.

Inside the “chrome” folder, create or edit the “userChrome.css” file and add the following

/ hides the native tabs /
#TabsToolbar {
visibility: collapse;
}/ leaves space for the window buttons /
#nav-bar {
margin-top: -8px;
margin-right: 74px;
margin-bottom: -4px;
}