gecko/browser/base/content/test/general/head_plain.js

function getTestPlugin(pluginName) {
  var ph = SpecialPowers.Cc["@mozilla.org/plugin/host;1"]
                                 .getService(SpecialPowers.Ci.nsIPluginHost);
  var tags = ph.getPluginTags();
  var name = pluginName || "Test Plug-in";
  for (var tag of tags) {
    if (tag.name == name) {
      return tag;
    }
  }

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

// call this to set the test plugin(s) initially expected enabled state.
// it will automatically be reset to it's previous value after the test
// ends
function setTestPluginEnabledState(newEnabledState, pluginName) {
  var plugin = getTestPlugin(pluginName);
  var oldEnabledState = plugin.enabledState;
  plugin.enabledState = newEnabledState;
  SimpleTest.registerCleanupFunction(function() {
    getTestPlugin(pluginName).enabledState = oldEnabledState;
  });
}