~/.mozilla/firefox/<id>.default/chrome/userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
  
#TabsToolbar {
    visibility: collapse !important;
}

#sidebar-header {
  display: none;
}

Before you close and relaunch, you must open about:config and toggle the property toolkit.legacyUserProfileCustomizations.stylesheets to true for this to be enabled.