Inside your profile folder, open or create chrome/userChrome.css

    /* to hide the native tabs */
    #TabsToolbar {
        visibility: collapse;
    }
    
    /* to hide the sidebar header */
    #sidebar-header {
        visibility: collapse;
    }

TreeStyleTabs author lists some more elaborate css [1].

[1] https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...

I remove the horizontal tab bar in all of my Firefox profiles:

https://superuser.com/questions/1424478/can-i-hide-native-ta...