If you are reading this probably you are like me. You hate the new design like me because is not compact and the tabs are not like tabs.
The previous design was fantastic, compact and very lite.
The new is basically strange and completely different like also for menu.
So how it is possible as a WebExtension theme cannot be so advanced? It is possible using userChrome.css and userContent.css files that you can insert in your mozilla profile folder.
PS: I have in userChrome.css also another snippet that add in the tab also the tab number so it is more easy to switch with ALT+(number) to change the tab.