You'll need a custom userChrome.css

    @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

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

You probably want to enable the menu bar if you do this, to stop the minimize/maximize/close buttons from overlapping the menu & other buttons on the right top corner.

There is no OOB way to do it ATM, you need to modify the userChrome.css. see below for more details.

https://www.reddit.com/r/firefox/comments/736cji/how_to_hide...