/* hides the native tabs */
#TabsToolbar {
    visibility: collapse;
}

#sidebar-header {
    visibility: collapse !important;
}

#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
  opacity: 0;
  pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse !important;
}

Update userchrome for better tab behavior
Update userchrome for better tab behavior