gecko-dev/devtools/client/themes/scratchpad.css

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#scratchpad-sidebar > tabs {
  height: 0;
  border: none;
}

#sp-toolbar {
  border: none;
}