The Tor Browser / file revision

 // this automatically sets the test plugin to be enabled (not e.g. click-to-play)

 // and resets this afterwards

 (function() {

   function getTestPlugin(aPluginName) {

     var ph = SpecialPowers.Cc["@mozilla.org/plugin/host;1"]

                           .getService(SpecialPowers.Ci.nsIPluginHost);

     var tags = ph.getPluginTags();

     for (var tag of tags) {

       if (tag.name == aPluginName) {

        return tag;

       }

     }

     ok(false, "Could not find plugin tag with plugin name '" + name + "'");

     return null;

   }

   var plugin = getTestPlugin("Test Plug-in");

   var oldEnabledState = plugin.enabledState;

   plugin.enabledState = SpecialPowers.Ci.nsIPluginTag.STATE_ENABLED;

   SimpleTest.registerCleanupFunction(function() {

     getTestPlugin("Test Plug-in").enabledState = oldEnabledState;

   });

 })();