title: FireFox "Software installation has been disabled by your system administrator"
body: <p>Installing Firebug in FireFox I get this error message:</p>
  
  <blockquote>
    <p>Software installation has been disabled by your system administrator</p>
  </blockquote>
  
  <p>I am a administrator of this PC, how do I allow installation of Firefox addons?</p>
  
  <p><sub>Note: answering my own problem to help others in the future.</sub></p>