new file mode 100644
--- /dev/null
+++ b/mail/components/debugger/Makefile.in
@@ -0,0 +1,12 @@
+# 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/.
+
+DEPTH     = @DEPTH@
+topsrcdir = @top_srcdir@
+srcdir    = @srcdir@
+VPATH     = @srcdir@
+
+include $(DEPTH)/config/autoconf.mk
+
+include $(topsrcdir)/config/rules.mk

new file mode 100644
--- /dev/null
+++ b/mail/components/debugger/content/dbg-mail-actors.js
@@ -0,0 +1,59 @@
+/* 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/. */
+
+Components.utils.import("resource://gre/modules/iteratorUtils.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+/**
+ * Create the root actor for Thunderbird's debugger implementation.
+ *
+ * @param aConnection       The debugger connection to create the actor for.
+ * @return                  The mail actor for the connection.
+ */
+function createRootActor(aConnection) {
+  let parameters = {
+    tabList: new MailTabList(aConnection),
+    globalActorFactories: DebuggerServer.globalActorFactories,
+    onShutdown: sendShutdownEvent,
+  };
+
+  let mailActor = new RootActor(aConnection, parameters);
+  mailActor.applicationType = "mail";
+  return mailActor;
+}
+
+/**
+ * Send a debugger shutdown event to all mail windows.
+ */
+function sendShutdownEvent() {
+  for (let win in fixIterator(Services.wm.getEnumerator("mail:3pane"))) {
+    let evt = win.document.createEvent("Event");
+    evt.initEvent("Debugger:Shutdown", true, false);
+    win.document.documentElement.dispatchEvent(evt);
+  }
+}
+
+/**
+ * The live list of tabs for Thunderbird. The term tab is taken from Firefox
+ * tabs, where each browser tab shows up as a tab in the debugger. As in
+ * Thunderbird all tabs are chrome tabs, we will be iterating the content
+ * windows and presenting them as tabs instead.
+ *
+ * @param aConnection       The connection to create the tab list for
+ */
+function MailTabList(aConnection) {
+  BrowserTabList.call(this, aConnection);
+}
+
+MailTabList.prototype = {
+  __proto__: BrowserTabList.prototype,
+
+  iterator: function() {
+    // For now we just need to make sure this function is a generator. Actually
+    // returning a list of tabs will be done in a future bug.
+    for (let dummy in []) {
+      yield dummy;
+    }
+  }
+};

new file mode 100644
--- /dev/null
+++ b/mail/components/debugger/content/dbg-messenger-overlay.js
@@ -0,0 +1,103 @@
+/* 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/. */
+
+Components.utils.import("resource://gre/modules/devtools/dbg-server.jsm");
+Components.utils.import("resource://gre/modules/XPCOMUtils.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "console", "resource://gre/modules/devtools/Console.jsm");
+
+/**
+ * Start the devtools debugger server and open a listener to contact it.
+ */
+function startDebugger() {
+  if (!DebuggerServer.initialized) {
+    // Initialize the debugger, if non-local connections are permitted then
+    // have the default prompt kick in.
+    DebuggerServer.init(() => {
+      return Services.prefs.getBoolPref("devtools.debugger.force-local") ||
+             DebuggerServer._defaultAllowConnection();
+    });
+
+    // Load the toolkit actor first
+    DebuggerServer.addActors("resource://gre/modules/devtools/server/actors/webbrowser.js");
+
+    // Add the mail root actor and the chrome debugger global actor
+    DebuggerServer.addActors("chrome://messenger/content/debugger/dbg-mail-actors.js");
+    DebuggerServer.addActors("resource://gre/modules/devtools/server/actors/script.js");
+    DebuggerServer.addGlobalActor(DebuggerServer.ChromeDebuggerActor, "chromeDebugger");
+  }
+
+  // Start the debugger listener unconditionally, it will check itself if it
+  // really needs to be started.
+  let port = Services.prefs.getIntPref('devtools.debugger.remote-port') || 6000;
+  try {
+    DebuggerServer.openListener(port);
+  } catch (e) {
+    console.exception("Unable to start debugger server", e);
+  }
+}
+
+/**
+ * Quit the devtools debugger server, forcing to disconnect all connections.
+ */
+function stopDebugger() {
+  try {
+    DebuggerServer.closeListener(true);
+  } catch (e) {
+    console.exception("Unable to stop debugger server", e);
+  }
+}
+
+/**
+ * Handler to call when the checked state of the menuitem is toggled. Sets the
+ * remote-enabled preference of the debugger and starts the debugger if needed.
+ */
+function toggleDebugger() {
+  let shouldEnable = document.getElementById("devtoolsDebugger").getAttribute("checked") == "true";
+  Services.prefs.setBoolPref("devtools.debugger.remote-enabled", shouldEnable);
+
+  if (shouldEnable) {
+    startDebugger();
+  } else {
+    stopDebugger();
+  }
+}
+
+/**
+ * Intialize the checked state, to be used when the view menu is opened.
+ */
+function initDebuggerToolsMenu() {
+  let debuggerEnabled = Services.prefs.getBoolPref("devtools.debugger.remote-enabled");
+  document.getElementById("devtoolsDebugger")
+          .setAttribute("checked", debuggerEnabled);
+}
+
+/**
+ * Intialize everything needed on load in the window for the debugger, i.e
+ * menuitem checked state listeners.
+ */
+function loadDebugger() {
+  window.removeEventListener("load", loadDebugger, false);
+  let viewPopup = document.getElementById("taskPopup");
+  viewPopup.addEventListener("popupshowing", initDebuggerToolsMenu, false);
+
+  // Call these functions once to start or stop the debugger on startup.
+  initDebuggerToolsMenu();
+  toggleDebugger();
+}
+
+/**
+ * Shutdown everything needed on load in the window for the debugger, i.e
+ * menuitem checked state listeners.
+ */
+function unloadDebugger() {
+  window.removeEventListener("unload", unloadDebugger, false);
+  let viewPopup = document.getElementById("taskPopup");
+  viewPopup.removeEventListener("popupshowing", initDebuggerToolsMenu, false);
+}
+
+// Load and unload the debugger when the window loads/unloads.
+window.addEventListener("load", loadDebugger, false);
+window.addEventListener("unload", unloadDebugger, false);

new file mode 100644
--- /dev/null
+++ b/mail/components/debugger/content/dbg-messenger-overlay.xul
@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- 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/. -->
+
+<!DOCTYPE overlay SYSTEM "chrome://messenger/locale/messenger.dtd">
+
+<overlay id="debugger-overlay"
+         xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+  <script type="application/javascript" src="chrome://messenger/content/debugger/dbg-messenger-overlay.js"/>
+
+  <menupopup id="taskPopup">
+    <menuitem id="devtoolsDebugger"
+              type="checkbox"
+              insertafter="menu_inspector,javascriptConsole,javaScriptConsole"
+              label="&allowRemoteDebugging.label;"
+              accesskey="&allowRemoteDebugging.accesskey;"
+              oncommand="toggleDebugger()"/>
+  </menupopup>
+</overlay>

new file mode 100644
--- /dev/null
+++ b/mail/components/debugger/jar.mn
@@ -0,0 +1,9 @@
+# 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/.
+
+messenger.jar:
+% overlay chrome://messenger/content/messenger.xul chrome://messenger/content/debugger/dbg-messenger-overlay.xul
+  content/messenger/debugger/dbg-messenger-overlay.xul     (content/dbg-messenger-overlay.xul)
+  content/messenger/debugger/dbg-messenger-overlay.js      (content/dbg-messenger-overlay.js)
+  content/messenger/debugger/dbg-mail-actors.js            (content/dbg-mail-actors.js)

new file mode 100644
--- /dev/null
+++ b/mail/components/debugger/moz.build
@@ -0,0 +1,4 @@
+# vim: set filetype=python:
+# 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/.

--- a/mail/components/moz.build
+++ b/mail/components/moz.build
@@ -3,16 +3,17 @@
 # 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/.
 
 # Only Mac and Windows have search integration components, but we include at
 # least one module from search/ on all platforms
 DIRS += [
     'compose',
     'cloudfile',
+    'debugger',
     'preferences',
     'addrbook',
     'migration',
     'activity',
     'search',
     'about-support',
     'wintaskbar',
     'newmailaccount',

--- a/mail/locales/en-US/chrome/messenger/messenger.dtd
+++ b/mail/locales/en-US/chrome/messenger/messenger.dtd
@@ -482,16 +482,18 @@
 <!ENTITY errorConsoleCmd.commandkey "j">
 <!ENTITY clearRecentHistory.label "Clear Recent History…">
 <!ENTITY clearRecentHistory.accesskey "H">
 <!ENTITY accountManagerCmd.label "Account Settings…">
 <!ENTITY accountManagerCmd.accesskey "S">
 <!-- LOCALIZATION NOTE (accountManagerCmdUnix.accesskey): belongs to accountManagerCmd.label,
         which is placed under the Edit menu on Unix systems -->
 <!ENTITY accountManagerCmdUnix.accesskey "A">
+<!ENTITY allowRemoteDebugging.label "Allow Remote Debugging">
+<!ENTITY allowRemoteDebugging.accesskey "g">
 
 <!-- Mail Toolbar -->
 <!ENTITY getMsgButton.label "Get Mail">
 <!ENTITY newMsgButton.label "Write">
 <!ENTITY replyButton.label "Reply">
 <!ENTITY replyAllButton.label "Reply All">
 <!ENTITY replyListButton.label "Reply to List">
 <!ENTITY forwardButton.label "Forward">