--- a/devtools/shim/DevToolsShim.jsm
+++ b/devtools/shim/DevToolsShim.jsm
@@ -204,21 +204,17 @@ this.DevToolsShim = {
    * Called from SessionStore.jsm in mozilla-central when restoring a state that contained
    * opened scratchpad windows.
    */
   restoreScratchpadSession: function (scratchpads) {
     if (!this.isInstalled()) {
       return;
     }
 
-    if (!this.isInitialized()) {
-      this._initDevTools();
-    }
-
-    this._gDevTools.restoreScratchpadSession(scratchpads);
+    this.gDevTools.restoreScratchpadSession(scratchpads);
   },
 
   /**
    * Called from nsContextMenu.js in mozilla-central when using the Inspect Element
    * context menu item.
    *
    * @param {XULTab} tab
    *        The browser tab on which inspect node was used.

--- a/devtools/shim/tests/unit/test_devtools_shim.js
+++ b/devtools/shim/tests/unit/test_devtools_shim.js
@@ -1,20 +1,17 @@
 /* -*- js-indent-level: 2; indent-tabs-mode: nil -*- */
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 
 "use strict";
 
-const { DevToolsShim: realDevToolsShim } =
+const { DevToolsShim } =
     Components.utils.import("chrome://devtools-shim/content/DevToolsShim.jsm", {});
 
-// Create a copy of the DevToolsShim for the test.
-const DevToolsShim = Object.assign({}, realDevToolsShim);
-
 // Test the DevToolsShim
 
 /**
  * Create a mocked version of DevTools that records all calls made to methods expected
  * to be called by DevToolsShim.
  */
 function createMockDevTools() {
   let methods = [
@@ -39,21 +36,23 @@ function createMockDevTools() {
       mock.callLog[method].push(args);
     };
     mock.callLog[method] = [];
   }
 
   return mock;
 }
 
+let isInstalledMethodBackup = DevToolsShim.isInstalled;
 function mockDevToolsInstalled() {
   DevToolsShim.isInstalled = () => true;
 }
-function mockDevToolsUninstalled() {
-  DevToolsShim.isInstalled = () => false;
+
+function restoreDevToolsInstalled() {
+  DevToolsShim.isInstalled = isInstalledMethodBackup;
 }
 
 /**
  * Check if a given method was called an expected number of times, and finally check the
  * arguments provided to the last call, if appropriate.
  */
 function checkCalls(mock, method, length, lastArgs) {
   ok(mock.callLog[method].length === length,
@@ -67,71 +66,87 @@ function checkCalls(mock, method, length
   for (let i = 0; i < lastArgs.length; i++) {
     let expectedArg = lastArgs[i];
     ok(mock.callLog[method][length - 1][i] === expectedArg,
         `Devtools.${method} was called with the expected argument (index ${i})`);
   }
 }
 
 function test_register_unregister() {
+  mockDevToolsInstalled();
+
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   DevToolsShim.register(createMockDevTools());
   ok(DevToolsShim.isInitialized(), "DevTools are installed");
 
   DevToolsShim.unregister();
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
+
+  restoreDevToolsInstalled();
 }
 
 function test_on_is_forwarded_to_devtools() {
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   function cb1() {}
   function cb2() {}
   let mock = createMockDevTools();
 
   DevToolsShim.on("test_event", cb1);
   DevToolsShim.register(mock);
   checkCalls(mock, "on", 1, ["test_event", cb1]);
 
   DevToolsShim.on("other_event", cb2);
   checkCalls(mock, "on", 2, ["other_event", cb2]);
+
+  restoreDevToolsInstalled();
 }
 
 function test_off_called_before_registering_devtools() {
+  mockDevToolsInstalled();
+
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   function cb1() {}
   let mock = createMockDevTools();
 
   DevToolsShim.on("test_event", cb1);
   DevToolsShim.off("test_event", cb1);
 
   DevToolsShim.register(mock);
   checkCalls(mock, "on", 0);
+
+  restoreDevToolsInstalled();
 }
 
 function test_off_called_before_with_bad_callback() {
+  mockDevToolsInstalled();
+
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   function cb1() {}
   function cb2() {}
   let mock = createMockDevTools();
 
   DevToolsShim.on("test_event", cb1);
   DevToolsShim.off("test_event", cb2);
 
   DevToolsShim.register(mock);
   // on should still be called
   checkCalls(mock, "on", 1, ["test_event", cb1]);
   // Calls to off should not be held and forwarded.
   checkCalls(mock, "off", 0);
+
+  restoreDevToolsInstalled();
 }
 
 function test_registering_tool() {
+  mockDevToolsInstalled();
+
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   let tool1 = {};
   let tool2 = {};
   let tool3 = {};
   let mock = createMockDevTools();
 
   // Pre-register tool1
@@ -148,19 +163,23 @@ function test_registering_tool() {
   checkCalls(mock, "registerTool", 2, [tool2]);
 
   DevToolsShim.unregister();
 
   // Create a new mock and check the tools are not added once again.
   mock = createMockDevTools();
   DevToolsShim.register(mock);
   checkCalls(mock, "registerTool", 0);
+
+  restoreDevToolsInstalled();
 }
 
 function test_registering_theme() {
+  mockDevToolsInstalled();
+
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   let theme1 = {};
   let theme2 = {};
   let theme3 = {};
   let mock = createMockDevTools();
 
   // Pre-register theme1
@@ -177,37 +196,41 @@ function test_registering_theme() {
   checkCalls(mock, "registerTheme", 2, [theme2]);
 
   DevToolsShim.unregister();
 
   // Create a new mock and check the themes are not added once again.
   mock = createMockDevTools();
   DevToolsShim.register(mock);
   checkCalls(mock, "registerTheme", 0);
+
+  restoreDevToolsInstalled();
 }
 
 function test_events() {
+  mockDevToolsInstalled();
+
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   let mock = createMockDevTools();
   // Check emit was not called.
   checkCalls(mock, "emit", 0);
 
   // Check emit is called once with the devtools-registered event.
   DevToolsShim.register(mock);
   checkCalls(mock, "emit", 1, ["devtools-registered"]);
 
   // Check emit is called once with the devtools-unregistered event.
   DevToolsShim.unregister();
   checkCalls(mock, "emit", 2, ["devtools-unregistered"]);
+
+  restoreDevToolsInstalled();
 }
 
 function test_scratchpad_apis() {
-  mockDevToolsUninstalled();
-
   ok(!DevToolsShim.isInstalled(), "DevTools are not installed");
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
 
   // Ensure that getOpenedScratchpads doesn't initialize the tools
   DevToolsShim.getOpenedScratchpads();
 
   ok(!DevToolsShim.isInstalled(), "DevTools are not installed");
   ok(!DevToolsShim.isInitialized(), "DevTools are not initialized");
@@ -234,16 +257,18 @@ function test_scratchpad_apis() {
   let scratchpadSessions = [{}];
   DevToolsShim.restoreScratchpadSession(scratchpadSessions);
   checkCalls(mock, "restoreScratchpadSession", 1, [scratchpadSessions]);
 
   ok(DevToolsShim.isInitialized(), "DevTools are initialized");
 
   DevToolsShim.getOpenedScratchpads();
   checkCalls(mock, "getOpenedScratchpads", 1, []);
+
+  restoreDevToolsInstalled();
 }
 
 function run_test() {
   test_register_unregister();
   DevToolsShim.unregister();
 
   test_on_is_forwarded_to_devtools();
   DevToolsShim.unregister();