GNOME theme for system integration, and the most important window altering changes I've made is to install Tree Style Tabs and hiding the top bar through 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;
    }