--- a/devtools/client/debugger/test/mochitest/browser_dbg_breakpoints-other-tabs.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg_breakpoints-other-tabs.js
@@ -32,11 +32,10 @@ var test = Task.async(function* () {
   callInTab(tab2, "testCase");
   const packet = yield paused;
 
   is(packet.why.type, "debuggerStatement",
      "Should have stopped at the debugger statement, not the other tab's breakpoint");
   is(packet.frame.where.line, 3,
      "Should have stopped at line 3 (debugger statement), not line 2 (other tab's breakpoint)");
 
-  yield teardown(panel1);
   yield resumeDebuggerThenCloseAndFinish(panel2);
 });

--- a/devtools/client/debugger/test/mochitest/browser_dbg_host-layout.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg_host-layout.js
@@ -79,34 +79,34 @@ function getHost(host) {
   if (host.indexOf("window") == 0) {
     return "window";
   }
   return host;
 }
 
 function resizeToolboxWindow(panel, host) {
   let sizeOption = host.split(":")[1];
-  let win = panel._toolbox._host._window;
+  let win = panel._toolbox.win.parent;
 
   // should be the same value as BREAKPOINT_SMALL_WINDOW_WIDTH in debugger-view.js
   let breakpoint = 850;
 
   if (sizeOption == "big" && win.outerWidth <= breakpoint) {
     yield resizeAndWaitForLayoutChange(panel, breakpoint + 300);
   } else if (sizeOption == "small" && win.outerWidth >= breakpoint) {
     yield resizeAndWaitForLayoutChange(panel, breakpoint - 300);
   } else {
     info("Window resize unnecessary for host " + host);
   }
 }
 
 function resizeAndWaitForLayoutChange(panel, width) {
     info("Updating toolbox window width to " + width);
 
-    let win = panel._toolbox._host._window;
+    let win = panel._toolbox.win.parent;
     let gDebugger = panel.panelWin;
 
     win.resizeTo(width, window.screen.availHeight);
     yield waitEventOnce(gDebugger, gDebugger.EVENTS.LAYOUT_CHANGED);
 }
 
 function testHost(aPanel, aHostType, aLayoutType) {
   let gDebugger = aPanel.panelWin;

--- a/devtools/client/debugger/test/mochitest/browser_dbg_worker-window.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg_worker-window.js
@@ -29,18 +29,27 @@ add_task(function* () {
   let { workers } = yield listWorkers(tabClient);
   let [, workerClient] = yield attachWorker(tabClient,
                                              findWorker(workers, WORKER_URL));
 
   let toolbox = yield gDevTools.showToolbox(TargetFactory.forWorker(workerClient),
                                             "jsdebugger",
                                             Toolbox.HostType.WINDOW);
 
-  is(toolbox._host.type, "window", "correct host");
-  ok(toolbox._host._window.document.title.includes(WORKER_URL),
+  is(toolbox.hostType, "window", "correct host");
+
+  yield new Promise(done => {
+    toolbox.win.parent.addEventListener("message", function onmessage(event) {
+      if (event.data.name == "set-host-title") {
+        toolbox.win.parent.removeEventListener("message", onmessage);
+        done();
+      }
+    });
+  });
+  ok(toolbox.win.parent.document.title.includes(WORKER_URL),
      "worker URL in host title");
 
   let toolTabs = toolbox.doc.querySelectorAll(".devtools-tab");
   let activeTools = [...toolTabs].map(tab=>tab.getAttribute("toolid"));
 
   is(activeTools.join(","), "webconsole,jsdebugger,scratchpad,options",
     "Correct set of tools supported by worker");
 

--- a/devtools/client/debugger/test/mochitest/head.js
+++ b/devtools/client/debugger/test/mochitest/head.js
@@ -742,16 +742,19 @@ AddonDebugger.prototype = {
 
       groupmap.get(group).get(label).url = source.url.split(" -> ").pop();
     }
 
     return groups;
   }),
 
   _onMessage: function (event) {
+    if (typeof(event.data) !== "string") {
+      return;
+    }
     let json = JSON.parse(event.data);
     switch (json.name) {
       case "toolbox-title":
         this.title = json.data.value;
         break;
     }
   }
 };

--- a/devtools/client/framework/test/browser_devtools_api.js
+++ b/devtools/client/framework/test/browser_devtools_api.js
@@ -66,17 +66,17 @@ function runTests1(aTab) {
     toolbox.once(toolId1 + "-ready", (event, panel) => {
       ok(panel, "panel argument available");
       events["ready"] = true;
     });
   });
 
   gDevTools.showToolbox(target, toolId1).then(function (toolbox) {
     is(toolbox.target, target, "toolbox target is correct");
-    is(toolbox._host.hostTab, gBrowser.selectedTab, "toolbox host is correct");
+    is(toolbox.target.tab, gBrowser.selectedTab, "targeted tab is correct");
 
     ok(events["init"], "init event fired");
     ok(events["ready"], "ready event fired");
 
     gDevTools.unregisterTool(toolId1);
 
     // Wait for unregisterTool to select the next tool before calling runTests2,
     // otherwise we will receive the wrong select event when waiting for
@@ -134,17 +134,17 @@ function runTests2() {
     toolbox.once(toolId2 + "-ready", (event, panel) => {
       ok(panel, "panel argument available");
       events["ready"] = true;
     });
   });
 
   gDevTools.showToolbox(target, toolId2).then(function (toolbox) {
     is(toolbox.target, target, "toolbox target is correct");
-    is(toolbox._host.hostTab, gBrowser.selectedTab, "toolbox host is correct");
+    is(toolbox.target.tab, gBrowser.selectedTab, "targeted tab is correct");
 
     ok(events["init"], "init event fired");
     ok(events["build"], "build event fired");
     ok(events["ready"], "ready event fired");
 
     continueTests(toolbox);
   });
 }

--- a/devtools/client/framework/test/browser_toolbox_custom_host.js
+++ b/devtools/client/framework/test/browser_toolbox_custom_host.js
@@ -22,16 +22,19 @@ function test() {
     gDevTools.showToolbox(target, null, Toolbox.HostType.CUSTOM, options)
              .then(testCustomHost, console.error)
              .then(null, console.error);
   }, true);
 
   content.location = "data:text/html,test custom host";
 
   function onMessage(event) {
+    if (typeof(event.data) !== "string") {
+      return;
+    }
     info("onMessage: " + event.data);
     let json = JSON.parse(event.data);
     if (json.name == "toolbox-close") {
       ok("Got the `toolbox-close` message");
       window.removeEventListener("message", onMessage);
       cleanup();
     }
   }

--- a/devtools/client/framework/test/browser_toolbox_raise.js
+++ b/devtools/client/framework/test/browser_toolbox_raise.js
@@ -40,35 +40,35 @@ function testBottomHost(aToolbox) {
 
 function testWindowHost() {
   // Make sure toolbox is not focused.
   window.addEventListener("focus", onFocus, true);
 
   // Need to wait for focus  as otherwise window.focus() is overridden by
   // toolbox window getting focused first on Linux and Mac.
   let onToolboxFocus = () => {
-    toolbox._host._window.removeEventListener("focus", onToolboxFocus, true);
+    toolbox.win.parent.removeEventListener("focus", onToolboxFocus, true);
     info("focusing main window.");
     window.focus();
   };
   // Need to wait for toolbox window to get focus.
-  toolbox._host._window.addEventListener("focus", onToolboxFocus, true);
+  toolbox.win.parent.addEventListener("focus", onToolboxFocus, true);
 }
 
 function onFocus() {
   info("Main window is focused before calling toolbox.raise()");
   window.removeEventListener("focus", onFocus, true);
 
   // Check if toolbox window got focus.
   let onToolboxFocusAgain = () => {
-    toolbox._host._window.removeEventListener("focus", onToolboxFocusAgain, false);
+    toolbox.win.parent.removeEventListener("focus", onToolboxFocusAgain, false);
     ok(true, "Toolbox window is the focused window after calling toolbox.raise()");
     cleanup();
   };
-  toolbox._host._window.addEventListener("focus", onToolboxFocusAgain, false);
+  toolbox.win.parent.addEventListener("focus", onToolboxFocusAgain, false);
 
   // Now raise toolbox.
   toolbox.raise();
 }
 
 function cleanup() {
   Services.prefs.setCharPref("devtools.toolbox.host", Toolbox.HostType.BOTTOM);
 

--- a/devtools/client/framework/test/browser_toolbox_toggle.js
+++ b/devtools/client/framework/test/browser_toolbox_toggle.js
@@ -77,17 +77,17 @@ function* testToggleDetachedToolbox(tab,
 
   let onMainWindowFocus = once(window, "focus");
   window.focus();
   yield onMainWindowFocus;
   ok(true, "Main window focused");
 
   info("Verify windowed toolbox is focused instead of closed when using " +
     "toggle key from the main window");
-  let toolboxWindow = toolbox._host._window;
+  let toolboxWindow = toolbox.win.top;
   let onToolboxWindowFocus = once(toolboxWindow, "focus", true);
   EventUtils.synthesizeKey(key, modifiers);
   yield onToolboxWindowFocus;
   ok(true, "Toolbox focused and not destroyed");
 
   info("Verify windowed toolbox is destroyed when using toggle key from its " +
     "own window");
 

--- a/devtools/client/framework/test/browser_toolbox_window_title_changes.js
+++ b/devtools/client/framework/test/browser_toolbox_window_title_changes.js
@@ -2,16 +2,28 @@
 /* vim: set ft=javascript ts=2 et sw=2 tw=80: */
 /* Any copyright is dedicated to the Public Domain.
  * http://creativecommons.org/publicdomain/zero/1.0/ */
 
 requestLongerTimeout(5);
 
 var {Toolbox} = require("devtools/client/framework/toolbox");
 
+// Wait for a given toolbox to get its title updated
+function waitForTitleChange(toolbox) {
+  let deferred = defer();
+  toolbox.win.parent.addEventListener("message", function onmessage(event) {
+    if (event.data.name == "set-host-title") {
+      toolbox.win.parent.removeEventListener("message", onmessage);
+      deferred.resolve();
+    }
+  });
+  return deferred.promise;
+}
+
 function test() {
   const URL_1 = "data:text/plain;charset=UTF-8,abcde";
   const URL_2 = "data:text/plain;charset=UTF-8,12345";
   const URL_3 = URL_ROOT + "browser_toolbox_window_title_changes_page.html";
 
   const TOOL_ID_1 = "webconsole";
   const TOOL_ID_2 = "jsdebugger";
 
@@ -23,55 +35,66 @@ function test() {
 
   addTab(URL_1).then(function () {
     let target = TargetFactory.forTab(gBrowser.selectedTab);
     gDevTools.showToolbox(target, null, Toolbox.HostType.BOTTOM)
       .then(function (aToolbox) { toolbox = aToolbox; })
       .then(() => toolbox.selectTool(TOOL_ID_1))
 
     // undock toolbox and check title
-      .then(() => toolbox.switchHost(Toolbox.HostType.WINDOW))
+      .then(() => {
+        // We have to first switch the host in order to spawn the new top level window
+        // on which we are going to listen from title change event
+        return toolbox.switchHost(Toolbox.HostType.WINDOW)
+          .then(() => waitForTitleChange(toolbox));
+      })
       .then(checkTitle.bind(null, NAME_1, URL_1, "toolbox undocked"))
 
     // switch to different tool and check title
-      .then(() => toolbox.selectTool(TOOL_ID_2))
+      .then(() => {
+        let onTitleChanged = waitForTitleChange(toolbox);
+        toolbox.selectTool(TOOL_ID_2);
+        return onTitleChanged;
+      })
       .then(checkTitle.bind(null, NAME_1, URL_1, "tool changed"))
 
     // navigate to different local url and check title
       .then(function () {
-        let deferred = defer();
-        target.once("navigate", () => deferred.resolve());
+        let onTitleChanged = waitForTitleChange(toolbox);
         gBrowser.loadURI(URL_2);
-        return deferred.promise;
+        return onTitleChanged;
       })
       .then(checkTitle.bind(null, NAME_2, URL_2, "url changed"))
 
     // navigate to a real url and check title
       .then(() => {
-        let deferred = defer();
-        target.once("navigate", () => deferred.resolve());
+        let onTitleChanged = waitForTitleChange(toolbox);
         gBrowser.loadURI(URL_3);
-        return deferred.promise;
+        return onTitleChanged;
       })
       .then(checkTitle.bind(null, NAME_3, URL_3, "url changed"))
 
     // destroy toolbox, create new one hosted in a window (with a
     // different tool id), and check title
       .then(function () {
         // Give the tools a chance to handle the navigation event before
         // destroying the toolbox.
         executeSoon(function () {
           toolbox.destroy()
             .then(function () {
               // After destroying the toolbox, a fresh target is required.
               target = TargetFactory.forTab(gBrowser.selectedTab);
               return gDevTools.showToolbox(target, null, Toolbox.HostType.WINDOW);
             })
             .then(function (aToolbox) { toolbox = aToolbox; })
-            .then(() => toolbox.selectTool(TOOL_ID_1))
+            .then(() => {
+              let onTitleChanged = waitForTitleChange(toolbox);
+              toolbox.selectTool(TOOL_ID_1);
+              return onTitleChanged;
+            })
             .then(checkTitle.bind(null, NAME_3, URL_3,
                                   "toolbox destroyed and recreated"))
 
             // clean up
             .then(() => toolbox.destroy())
             .then(function () {
               toolbox = null;
               gBrowser.removeCurrentTab();

--- a/devtools/client/framework/test/browser_toolbox_window_title_frame_select.js
+++ b/devtools/client/framework/test/browser_toolbox_window_title_frame_select.js
@@ -20,18 +20,24 @@ const IFRAME_URL = URL_ROOT + "browser_t
 add_task(function* () {
   Services.prefs.setBoolPref("devtools.command-button-frames.enabled", true);
 
   yield addTab(URL);
   let target = TargetFactory.forTab(gBrowser.selectedTab);
   let toolbox = yield gDevTools.showToolbox(target, null,
     Toolbox.HostType.BOTTOM);
 
+  let onTitleChanged = waitForTitleChange(toolbox);
   yield toolbox.selectTool("inspector");
+  yield onTitleChanged;
+
   yield toolbox.switchHost(Toolbox.HostType.WINDOW);
+  // Wait for title change event *after* switch host, in order to listen
+  // for the event on the WINDOW host window, which only exists after switchHost
+  yield waitForTitleChange(toolbox);
 
   is(getTitle(), `Developer Tools - Page title - ${URL}`,
     "Devtools title correct after switching to detached window host");
 
   // Wait for tick to avoid unexpected 'popuphidden' event, which
   // blocks the frame popup menu opened below. See also bug 1276873
   yield waitForTick();
 
@@ -51,24 +57,27 @@ add_task(function* () {
   let topFrameBtn = frames.filter(b => b.label == URL)[0];
   let iframeBtn = frames.filter(b => b.label == IFRAME_URL)[0];
   ok(topFrameBtn, "Got top level document in the list");
   ok(iframeBtn, "Got iframe document in the list");
 
   // Listen to will-navigate to check if the view is empty
   let willNavigate = toolbox.target.once("will-navigate");
 
+  onTitleChanged = waitForTitleChange(toolbox);
+
   // Only select the iframe after we are able to select an element from the top
   // level document.
   let newRoot = toolbox.getPanel("inspector").once("new-root");
   info("Select the iframe");
   iframeBtn.click();
 
   yield willNavigate;
   yield newRoot;
+  yield onTitleChanged;
 
   info("Navigation to the iframe is done, the inspector should be back up");
   is(getTitle(), `Developer Tools - Page title - ${URL}`,
     "Devtools title was not updated after changing inspected frame");
 
   info("Cleanup toolbox and test preferences.");
   yield toolbox.destroy();
   toolbox = null;
@@ -78,8 +87,20 @@ add_task(function* () {
   Services.prefs.clearUserPref("devtools.toolbox.sideEnabled");
   Services.prefs.clearUserPref("devtools.command-button-frames.enabled");
   finish();
 });
 
 function getTitle() {
   return Services.wm.getMostRecentWindow("devtools:toolbox").document.title;
 }
+
+// Wait for a given toolbox to get its title updated
+function waitForTitleChange(toolbox) {
+  let deferred = defer();
+  toolbox.win.parent.addEventListener("message", function onmessage(event) {
+    if (event.data.name == "set-host-title") {
+      toolbox.win.parent.removeEventListener("message", onmessage);
+      deferred.resolve();
+    }
+  });
+  return deferred.promise;
+}

--- a/devtools/client/inspector/test/browser_inspector_breadcrumbs_visibility.js
+++ b/devtools/client/inspector/test/browser_inspector_breadcrumbs_visibility.js
@@ -34,17 +34,17 @@ const NODES = [
 add_task(function* () {
   let { inspector, toolbox } = yield openInspectorForURL(TEST_URI);
 
   // No way to wait for scrolling to end (Bug 1172171)
   // Rather than wait a max time; limit test to instant scroll behavior
   inspector.breadcrumbs.arrowScrollBox.scrollBehavior = "instant";
 
   yield toolbox.switchHost(Toolbox.HostType.WINDOW);
-  let hostWindow = toolbox._host._window;
+  let hostWindow = toolbox.win.parent;
   let originalWidth = hostWindow.outerWidth;
   let originalHeight = hostWindow.outerHeight;
   hostWindow.resizeTo(640, 300);
 
   info("Testing transitions ltr");
   yield pushPref("intl.uidirection.en-US", "ltr");
   yield testBreadcrumbTransitions(hostWindow, inspector);
 

--- a/devtools/client/inspector/test/browser_inspector_pane-toggle-04.js
+++ b/devtools/client/inspector/test/browser_inspector_pane-toggle-04.js
@@ -20,17 +20,17 @@ add_task(function* () {
 
   let { inspector, toolbox } = yield openInspectorForURL("about:blank");
   let button = inspector.panelDoc.querySelector(".sidebar-toggle");
   let panel = inspector.panelDoc.querySelector("#inspector-sidebar-container");
 
   info("Changing toolbox host to a window.");
   yield toolbox.switchHost(Toolbox.HostType.WINDOW);
 
-  let hostWindow = toolbox._host._window;
+  let hostWindow = toolbox.win.parent;
   let originalWidth = hostWindow.outerWidth;
   let originalHeight = hostWindow.outerHeight;
 
   info("Resizing window to switch to the horizontal layout.");
   hostWindow.resizeTo(800, 300);
 
   // Check the sidebar is expanded when the test starts.
   ok(!panel.classList.contains("pane-collapsed"), "The panel is in expanded state");

--- a/devtools/client/styleeditor/test/browser_styleeditor_sv_resize.js
+++ b/devtools/client/styleeditor/test/browser_styleeditor_sv_resize.js
@@ -16,17 +16,17 @@ add_task(function* () {
   is(ui.editors.length, 2, "There are 2 style sheets initially");
 
   info("Changing toolbox host to a window.");
   yield toolbox.switchHost(Toolbox.HostType.WINDOW);
 
   let editor = yield ui.editors[0].getSourceEditor();
   let originalSourceEditor = editor.sourceEditor;
 
-  let hostWindow = toolbox._host._window;
+  let hostWindow = toolbox.win.parent;
   let originalWidth = hostWindow.outerWidth;
   let originalHeight = hostWindow.outerHeight;
 
   // to check the caret is preserved
   originalSourceEditor.setCursor(originalSourceEditor.getPosition(4));
 
   info("Resizing window.");
   hostWindow.resizeTo(120, 480);

--- a/devtools/client/webconsole/test/browser_webconsole_closure_inspection.js
+++ b/devtools/client/webconsole/test/browser_webconsole_closure_inspection.js
@@ -85,10 +85,10 @@ function onGetNameFetch(evt, view) {
 }
 
 function onExpandClosure(results) {
   let prop = results[0].matchedProp;
   ok(prop, "matched the name property in the variables view");
 
   gVariablesView.window.focus();
   gJSTerm.once("sidebar-closed", finishTest);
-  EventUtils.synthesizeKey("VK_ESCAPE", {});
+  EventUtils.synthesizeKey("VK_ESCAPE", {}, gVariablesView.window);
 }