gecko-dev/toolkit/content/customizeToolbar.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/. */

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
@namespace html url("http://www.w3.org/1999/xhtml"); /* namespace for HTML elements */

#palette-box {
  overflow: auto;
  display: block;
  min-height: 3em;
}

#palette-box > toolbarpaletteitem {
  width: 110px;
  height: 94px;
  overflow: hidden;
  display: inline-block;
}

.toolbarpaletteitem-box {
  -moz-box-pack: center;
  -moz-box-flex: 1;
  width: 110px;
  max-width: 110px;
}

toolbarpaletteitem > label {
  text-align: center;
}

#main-box > box {
  overflow: hidden; 
}

/* Hide the toolbarbutton label because we replicate it on the wrapper */
.toolbarbutton-text {
  display: none;
}