userChrome.css for some aspects of the Firefox >= 57 UI which can't be tweaked via regular config

userChrome.css

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

/* Remove the gap to the left of the title tab bar */
#TabsToolbar hbox.titlebar-placeholder[type="pre-tabs"] {
  display: none !important;
}

/* Remove the buttons on the right of the location bar */
#page-action-buttons {
  display: none !important;
}