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diff --git a/testing/mochitest/tests/SimpleTest/MockObjects.js b/testing/mochitest/tests/SimpleTest/MockObjects.js
new file mode 100644
index 0000000000..5782707c04
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+++ b/testing/mochitest/tests/SimpleTest/MockObjects.js
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+/* 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/. */
+
+/**
+ * Allows registering a mock XPCOM component, that temporarily replaces the
+ *  original one when an object implementing a given ContractID is requested
+ *  using createInstance.
+ *
+ * @param aContractID
+ *        The ContractID of the component to replace, for example
+ *        "@mozilla.org/filepicker;1".
+ *
+ * @param aReplacementCtor
+ *        The constructor function for the JavaScript object that will be
+ *        created every time createInstance is called. This object must
+ *        implement QueryInterface and provide the XPCOM interfaces required by
+ *        the specified ContractID (for example
+ *        Components.interfaces.nsIFilePicker).
+ */
+
+function MockObjectRegisterer(aContractID, aReplacementCtor) {
+  this._contractID = aContractID;
+  this._replacementCtor = aReplacementCtor;
+}
+
+MockObjectRegisterer.prototype = {
+  /**
+   * Replaces the current factory with one that returns a new mock object.
+   *
+   * After register() has been called, it is mandatory to call unregister() to
+   * restore the original component. Usually, you should use a try-catch block
+   * to ensure that unregister() is called.
+   */
+  register: function MOR_register() {
+    if (this._originalCID) {
+      throw new Error("Invalid object state when calling register()");
+    }
+
+    // Define a factory that creates a new object using the given constructor.
+    var isChrome = location.protocol == "chrome:";
+    var providedConstructor = this._replacementCtor;
+    this._mockFactory = {
+      createInstance: function MF_createInstance(aIid) {
+        var inst = new providedConstructor().QueryInterface(aIid);
+        if (!isChrome) {
+          inst = SpecialPowers.wrapCallbackObject(inst);
+        }
+        return inst;
+      },
+    };
+    if (!isChrome) {
+      this._mockFactory = SpecialPowers.wrapCallbackObject(this._mockFactory);
+    }
+
+    var retVal = SpecialPowers.swapFactoryRegistration(
+      null,
+      this._contractID,
+      this._mockFactory
+    );
+    if ("error" in retVal) {
+      throw new Error("ERROR: " + retVal.error);
+    } else {
+      this._originalCID = retVal.originalCID;
+    }
+  },
+
+  /**
+   * Restores the original factory.
+   */
+  unregister: function MOR_unregister() {
+    if (!this._originalCID) {
+      throw new Error("Invalid object state when calling unregister()");
+    }
+
+    // Free references to the mock factory.
+    SpecialPowers.swapFactoryRegistration(this._originalCID, this._contractID);
+
+    // Allow registering a mock factory again later.
+    this._originalCID = null;
+    this._mockFactory = null;
+  },
+
+  // --- Private methods and properties ---
+
+  /**
+   * The factory of the component being replaced.
+   */
+  _originalCID: null,
+
+  /**
+   * The nsIFactory that was automatically generated by this object.
+   */
+  _mockFactory: null,
+};