gecko-dev/themes/modern/communicator/brand.css

#navigator-throbber
  {
    -moz-binding          : url("chrome://global/content/xulBindings.xml#buttontop");
    list-style-image      : url("chrome://global/skin/animthrob_single.gif");
    margin                : 5px 1px 9px 8px;
    padding               : 0px;
    cursor                : pointer;
    border                : 0px;
    background-color	  : transparent;
    -moz-user-focus       : ignore;
  }  

#navigator-throbber[busy="true"]
  {
    list-style-image      : url("chrome://global/skin/animthrob.gif");
  }
  
#navigator-throbber:hover:active
  {
    margin               : 4px 0px 5px 9px;
  }

#navigator-throbber > .button-internal-box
  {
    border                : 0px;
    padding               : 0px;
    margin                : 0px;
  }
  
#navigator-throbber > .button-internal-box > .button-text-container
  {
    display               : none;
  }