--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -1437,19 +1437,20 @@ pref("devtools.debugger.remote-host", "l
 pref("devtools.debugger.remote-timeout", 20000);
 pref("devtools.debugger.pause-on-exceptions", false);
 pref("devtools.debugger.ignore-caught-exceptions", true);
 pref("devtools.debugger.source-maps-enabled", true);
 pref("devtools.debugger.pretty-print-enabled", true);
 pref("devtools.debugger.auto-pretty-print", false);
 pref("devtools.debugger.auto-black-box", true);
 pref("devtools.debugger.tracer", false);
+pref("devtools.debugger.workers", false);
 
 // The default Debugger UI settings
-pref("devtools.debugger.ui.panes-sources-width", 200);
+pref("devtools.debugger.ui.panes-workers-and-sources-width", 200);
 pref("devtools.debugger.ui.panes-instruments-width", 300);
 pref("devtools.debugger.ui.panes-visible-on-startup", false);
 pref("devtools.debugger.ui.variables-sorting-enabled", true);
 pref("devtools.debugger.ui.variables-only-enum-visible", false);
 pref("devtools.debugger.ui.variables-searchbox-visible", false);
 
 // Enable the Performance tools
 pref("devtools.performance.enabled", true);

--- a/browser/devtools/debugger/debugger-controller.js
+++ b/browser/devtools/debugger/debugger-controller.js
@@ -177,16 +177,19 @@ let DebuggerController = {
     if (this._shutdown) {
       return;
     }
 
     yield DebuggerView.destroy();
     this.SourceScripts.disconnect();
     this.StackFrames.disconnect();
     this.ThreadState.disconnect();
+    if (this._target.isTabActor) {
+      this.Workers.disconnect();
+    }
     this.Tracer.disconnect();
     this.disconnect();
 
     this._shutdown = true;
   }),
 
   /**
    * Initiates remote debugging based on the current target, wiring event
@@ -314,16 +317,17 @@ let DebuggerController = {
     };
 
     this._target.activeTab.attachThread(threadOptions, (aResponse, aThreadClient) => {
       if (!aThreadClient) {
         deferred.reject(new Error("Couldn't attach to thread: " + aResponse.error));
         return;
       }
       this.activeThread = aThreadClient;
+      this.Workers.connect();
       this.ThreadState.connect();
       this.StackFrames.connect();
       this.SourceScripts.connect();
 
       if (aThreadClient.paused) {
         aThreadClient.resume(this._ensureResumptionOrder);
       }
 
@@ -441,16 +445,85 @@ let DebuggerController = {
 
   _startup: false,
   _shutdown: false,
   _connected: false,
   client: null,
   activeThread: null
 };
 
+function Workers() {
+  this._workerClients = new Map();
+  this._onWorkerListChanged = this._onWorkerListChanged.bind(this);
+  this._onWorkerFreeze = this._onWorkerFreeze.bind(this);
+  this._onWorkerThaw = this._onWorkerThaw.bind(this);
+}
+
+Workers.prototype = {
+  get _tabClient() {
+    return DebuggerController._target.activeTab;
+  },
+
+  connect: function () {
+    if (!Prefs.workersEnabled) {
+      return;
+    }
+
+    this._updateWorkerList();
+    this._tabClient.addListener("workerListChanged", this._onWorkerListChanged);
+  },
+
+  disconnect: function () {
+    this._tabClient.removeListener("workerListChanged", this._onWorkerListChanged);
+  },
+
+  _updateWorkerList: function () {
+    this._tabClient.listWorkers((response) => {
+      let workerActors = new Set();
+      for (let worker of response.workers) {
+        workerActors.add(worker.actor);
+      }
+
+      for (let [workerActor, workerClient] of this._workerClients) {
+        if (!workerActors.has(workerActor)) {
+          workerClient.removeListener("freeze", this._onWorkerFreeze);
+          workerClient.removeListener("thaw", this._onWorkerThaw);
+          this._workerClients.delete(workerActor);
+          DebuggerView.Workers.removeWorker(workerActor);
+        }
+      }
+
+      for (let actor of workerActors) {
+        let workerActor = actor
+        if (!this._workerClients.has(workerActor)) {
+          this._tabClient.attachWorker(workerActor, (response, workerClient) => {
+            workerClient.addListener("freeze", this._onWorkerFreeze);
+            workerClient.addListener("thaw", this._onWorkerThaw);
+            this._workerClients.set(workerActor, workerClient);
+            DebuggerView.Workers.addWorker(workerActor, workerClient.url);
+          });
+        }
+      }
+    });
+  },
+
+  _onWorkerListChanged: function () {
+    this._updateWorkerList();
+  },
+
+  _onWorkerFreeze: function (type, packet) {
+    DebuggerView.Workers.removeWorker(packet.from);
+  },
+
+  _onWorkerThaw: function (type, packet) {
+    let workerClient = this._workerClients.get(packet.from);
+    DebuggerView.Workers.addWorker(packet.from, workerClient.url);
+  }
+};
+
 /**
  * ThreadState keeps the UI up to date with the state of the
  * thread (paused/attached/etc.).
  */
 function ThreadState() {
   this._update = this._update.bind(this);
   this.interruptedByResumeButton = false;
 }
@@ -2419,42 +2492,44 @@ HitCounts.prototype = {
  * Localization convenience methods.
  */
 let L10N = new ViewHelpers.L10N(DBG_STRINGS_URI);
 
 /**
  * Shortcuts for accessing various debugger preferences.
  */
 let Prefs = new ViewHelpers.Prefs("devtools", {
-  sourcesWidth: ["Int", "debugger.ui.panes-sources-width"],
+  workersAndSourcesWidth: ["Int", "debugger.ui.panes-workers-and-sources-width"],
   instrumentsWidth: ["Int", "debugger.ui.panes-instruments-width"],
   panesVisibleOnStartup: ["Bool", "debugger.ui.panes-visible-on-startup"],
   variablesSortingEnabled: ["Bool", "debugger.ui.variables-sorting-enabled"],
   variablesOnlyEnumVisible: ["Bool", "debugger.ui.variables-only-enum-visible"],
   variablesSearchboxVisible: ["Bool", "debugger.ui.variables-searchbox-visible"],
   pauseOnExceptions: ["Bool", "debugger.pause-on-exceptions"],
   ignoreCaughtExceptions: ["Bool", "debugger.ignore-caught-exceptions"],
   sourceMapsEnabled: ["Bool", "debugger.source-maps-enabled"],
   prettyPrintEnabled: ["Bool", "debugger.pretty-print-enabled"],
   autoPrettyPrint: ["Bool", "debugger.auto-pretty-print"],
   tracerEnabled: ["Bool", "debugger.tracer"],
+  workersEnabled: ["Bool", "debugger.workers"],
   editorTabSize: ["Int", "editor.tabsize"],
   autoBlackBox: ["Bool", "debugger.auto-black-box"]
 });
 
 /**
  * Convenient way of emitting events from the panel window.
  */
 EventEmitter.decorate(this);
 
 /**
  * Preliminary setup for the DebuggerController object.
  */
 DebuggerController.initialize();
 DebuggerController.Parser = new Parser();
+DebuggerController.Workers = new Workers();
 DebuggerController.ThreadState = new ThreadState();
 DebuggerController.StackFrames = new StackFrames();
 DebuggerController.SourceScripts = new SourceScripts();
 DebuggerController.Breakpoints = new Breakpoints();
 DebuggerController.Breakpoints.DOM = new EventListeners();
 DebuggerController.Tracer = new Tracer();
 DebuggerController.HitCounts = new HitCounts();
 

--- a/browser/devtools/debugger/debugger-view.js
+++ b/browser/devtools/debugger/debugger-view.js
@@ -50,16 +50,17 @@ let DebuggerView = {
     this._startup = deferred.promise;
 
     this._initializePanes();
     this.Toolbar.initialize();
     this.Options.initialize();
     this.Filtering.initialize();
     this.StackFrames.initialize();
     this.StackFramesClassicList.initialize();
+    this.Workers.initialize();
     this.Sources.initialize();
     this.VariableBubble.initialize();
     this.Tracer.initialize();
     this.WatchExpressions.initialize();
     this.EventListeners.initialize();
     this.GlobalSearch.initialize();
     this._initializeVariablesView();
     this._initializeEditor(deferred.resolve);
@@ -103,53 +104,53 @@ let DebuggerView = {
   /**
    * Initializes the UI for all the displayed panes.
    */
   _initializePanes: function() {
     dumpn("Initializing the DebuggerView panes");
 
     this._body = document.getElementById("body");
     this._editorDeck = document.getElementById("editor-deck");
-    this._sourcesPane = document.getElementById("sources-pane");
+    this._workersAndSourcesPane = document.getElementById("workers-and-sources-pane");
     this._instrumentsPane = document.getElementById("instruments-pane");
     this._instrumentsPaneToggleButton = document.getElementById("instruments-pane-toggle");
 
     this.showEditor = this.showEditor.bind(this);
     this.showBlackBoxMessage = this.showBlackBoxMessage.bind(this);
     this.showProgressBar = this.showProgressBar.bind(this);
     this.maybeShowBlackBoxMessage = this.maybeShowBlackBoxMessage.bind(this);
 
     this._onTabSelect = this._onInstrumentsPaneTabSelect.bind(this);
     this._instrumentsPane.tabpanels.addEventListener("select", this._onTabSelect);
 
     this._collapsePaneString = L10N.getStr("collapsePanes");
     this._expandPaneString = L10N.getStr("expandPanes");
 
-    this._sourcesPane.setAttribute("width", Prefs.sourcesWidth);
+    this._workersAndSourcesPane.setAttribute("width", Prefs.workersAndSourcesWidth);
     this._instrumentsPane.setAttribute("width", Prefs.instrumentsWidth);
     this.toggleInstrumentsPane({ visible: Prefs.panesVisibleOnStartup });
 
     // Side hosts requires a different arrangement of the debugger widgets.
     if (gHostType == "side") {
       this.handleHostChanged(gHostType);
     }
   },
 
   /**
    * Destroys the UI for all the displayed panes.
    */
   _destroyPanes: function() {
     dumpn("Destroying the DebuggerView panes");
 
     if (gHostType != "side") {
-      Prefs.sourcesWidth = this._sourcesPane.getAttribute("width");
+      Prefs.workersAndSourcesWidth = this._workersAndSourcesPane.getAttribute("width");
       Prefs.instrumentsWidth = this._instrumentsPane.getAttribute("width");
     }
 
-    this._sourcesPane = null;
+    this._workersAndSourcesPane = null;
     this._instrumentsPane = null;
     this._instrumentsPaneToggleButton = null;
   },
 
   /**
    * Initializes the VariablesView instance and attaches a controller.
    */
   _initializeVariablesView: function() {
@@ -608,17 +609,17 @@ let DebuggerView = {
    * Switches the debugger widgets to a horizontal layout.
    */
   _enterVerticalLayout: function() {
     let normContainer = document.getElementById("debugger-widgets");
     let vertContainer = document.getElementById("vertical-layout-panes-container");
 
     // Move the soruces and instruments panes in a different container.
     let splitter = document.getElementById("sources-and-instruments-splitter");
-    vertContainer.insertBefore(this._sourcesPane, splitter);
+    vertContainer.insertBefore(this._workersAndSourcesPane, splitter);
     vertContainer.appendChild(this._instrumentsPane);
 
     // Make sure the vertical layout container's height doesn't repeatedly
     // grow or shrink based on the displayed sources, variables etc.
     vertContainer.setAttribute("height",
       vertContainer.getBoundingClientRect().height);
   },
 
@@ -627,22 +628,22 @@ let DebuggerView = {
    */
   _enterHorizontalLayout: function() {
     let normContainer = document.getElementById("debugger-widgets");
     let vertContainer = document.getElementById("vertical-layout-panes-container");
 
     // The sources and instruments pane need to be inserted at their
     // previous locations in their normal container.
     let splitter = document.getElementById("sources-and-editor-splitter");
-    normContainer.insertBefore(this._sourcesPane, splitter);
+    normContainer.insertBefore(this._workersAndSourcesPane, splitter);
     normContainer.appendChild(this._instrumentsPane);
 
     // Revert to the preferred sources and instruments widths, because
     // they flexed in the vertical layout.
-    this._sourcesPane.setAttribute("width", Prefs.sourcesWidth);
+    this._workersAndSourcesPane.setAttribute("width", Prefs.workersAndSourcesWidth);
     this._instrumentsPane.setAttribute("width", Prefs.instrumentsWidth);
   },
 
   /**
    * Handles any initialization on a tab navigation event issued by the client.
    */
   handleTabNavigation: function() {
     dumpn("Handling tab navigation in the DebuggerView");
@@ -677,17 +678,17 @@ let DebuggerView = {
   VariableBubble: null,
   WatchExpressions: null,
   EventListeners: null,
   editor: null,
   _editorSource: {},
   _loadingText: "",
   _body: null,
   _editorDeck: null,
-  _sourcesPane: null,
+  _workersAndSourcesPane: null,
   _instrumentsPane: null,
   _instrumentsPaneToggleButton: null,
   _collapsePaneString: "",
   _expandPaneString: ""
 };
 
 /**
  * A custom items container, used for displaying views like the

--- a/browser/devtools/debugger/debugger.css
+++ b/browser/devtools/debugger/debugger.css
@@ -1,16 +1,17 @@
 /* -*- Mode: javascript; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* vim: set ft=javascript ts=2 et sw=2 tw=80: */
 /* 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/. */
 
 /* Side pane views */
 
+#workers-pane > tabpanels > tabpanel,
 #sources-pane > tabpanels > tabpanel,
 #instruments-pane > tabpanels > tabpanel {
   -moz-box-orient: vertical;
 }
 
 /* Toolbar controls */
 
 .devtools-toolbarbutton:not([label]) > .toolbarbutton-text {
@@ -18,17 +19,17 @@
 }
 
 /* Horizontal vs. vertical layout */
 
 #body[layout=vertical] #debugger-widgets {
   -moz-box-orient: vertical;
 }
 
-#body[layout=vertical] #sources-pane {
+#body[layout=vertical] #workers-and-sources-pane {
   -moz-box-flex: 1;
 }
 
 #body[layout=vertical] #instruments-pane {
   -moz-box-flex: 2;
 }
 
 #body[layout=vertical] #instruments-pane-toggle {

--- a/browser/devtools/debugger/debugger.xul
+++ b/browser/devtools/debugger/debugger.xul
@@ -23,16 +23,17 @@
         persist="screenX screenY width height sizemode">
 
   <script type="application/javascript;version=1.8"
           src="chrome://browser/content/devtools/theme-switching.js"/>
   <script type="text/javascript" src="chrome://global/content/globalOverlay.js"/>
   <script type="text/javascript" src="debugger-controller.js"/>
   <script type="text/javascript" src="debugger-view.js"/>
   <script type="text/javascript" src="debugger/utils.js"/>
+  <script type="text/javascript" src="debugger/workers-view.js"/>
   <script type="text/javascript" src="debugger/sources-view.js"/>
   <script type="text/javascript" src="debugger/variable-bubble-view.js"/>
   <script type="text/javascript" src="debugger/tracer-view.js"/>
   <script type="text/javascript" src="debugger/watch-expressions-view.js"/>
   <script type="text/javascript" src="debugger/event-listeners-view.js"/>
   <script type="text/javascript" src="debugger/global-search-view.js"/>
   <script type="text/javascript" src="debugger/toolbar-view.js"/>
   <script type="text/javascript" src="debugger/options-view.js"/>
@@ -307,71 +308,88 @@
                      class="devtools-toolbarbutton devtools-option-toolbarbutton"
                      tooltiptext="&debuggerUI.optsButton.tooltip;"
                      popup="debuggerPrefsContextMenu"
                      tabindex="0"/>
     </toolbar>
     <vbox id="globalsearch" orient="vertical" hidden="true"/>
     <splitter class="devtools-horizontal-splitter" hidden="true"/>
     <hbox id="debugger-widgets" flex="1">
-      <tabbox id="sources-pane"
-              class="devtools-sidebar-tabs">
-        <tabs>
-          <tab id="sources-tab" label="&debuggerUI.tabs.sources;"/>
-          <tab id="callstack-tab" label="&debuggerUI.tabs.callstack;"/>
-          <tab id="tracer-tab" label="&debuggerUI.tabs.traces;" hidden="true"/>
-        </tabs>
-        <tabpanels flex="1">
-          <tabpanel id="sources-tabpanel">
-            <vbox id="sources" flex="1"/>
-            <toolbar id="sources-toolbar" class="devtools-toolbar">
-              <hbox id="sources-controls"
-                    class="devtools-toolbarbutton-group">
-                <toolbarbutton id="black-box"
+      <vbox id="workers-and-sources-pane">
+        <tabbox id="workers-pane"
+                class="devtools-sidebar-tabs"
+                flex="0"
+                hidden="true">
+          <tabs>
+            <tab id="workers-tab" label="&debuggerUI.tabs.workers;"/>
+          </tabs>
+          <tabpanels flex="1">
+            <tabpanel>
+              <vbox id="workers" flex="1"/>
+            </tabpanel>
+          </tabpanels>
+        </tabbox>
+        <splitter class="devtools-horizontal-splitter"/>
+        <tabbox id="sources-pane"
+                class="devtools-sidebar-tabs"
+                flex="1">
+          <tabs>
+            <tab id="sources-tab" label="&debuggerUI.tabs.sources;"/>
+            <tab id="callstack-tab" label="&debuggerUI.tabs.callstack;"/>
+            <tab id="tracer-tab" label="&debuggerUI.tabs.traces;" hidden="true"/>
+          </tabs>
+          <tabpanels flex="1">
+            <tabpanel id="sources-tabpanel">
+              <vbox id="sources" flex="1"/>
+              <toolbar id="sources-toolbar" class="devtools-toolbar">
+                <hbox id="sources-controls"
+                      class="devtools-toolbarbutton-group">
+                  <toolbarbutton id="black-box"
+                                 class="devtools-toolbarbutton"
+                                 tooltiptext="&debuggerUI.sources.blackBoxTooltip;"
+                                 command="blackBoxCommand"/>
+                  <toolbarbutton id="pretty-print"
+                                 class="devtools-toolbarbutton"
+                                 tooltiptext="&debuggerUI.sources.prettyPrint;"
+                                 command="prettyPrintCommand"
+                                 hidden="true"/>
+                </hbox>
+                <vbox class="devtools-separator"/>
+                <toolbarbutton id="toggle-breakpoints"
                                class="devtools-toolbarbutton"
-                               tooltiptext="&debuggerUI.sources.blackBoxTooltip;"
-                               command="blackBoxCommand"/>
-                <toolbarbutton id="pretty-print"
-                               class="devtools-toolbarbutton"
-                               tooltiptext="&debuggerUI.sources.prettyPrint;"
-                               command="prettyPrintCommand"
-                               hidden="true"/>
+                               tooltiptext="&debuggerUI.sources.toggleBreakpoints;"
+                               command="toggleBreakpointsCommand"/>
+              </toolbar>
+            </tabpanel>
+            <tabpanel id="callstack-tabpanel">
+              <vbox id="callstack-list" flex="1"/>
+            </tabpanel>
+            <tabpanel id="tracer-tabpanel">
+              <vbox id="tracer-traces" flex="1"/>
+              <hbox class="trace-item-template" hidden="true">
+                <hbox class="trace-item" align="center" flex="1" crop="end">
+                  <label class="trace-type plain"/>
+                  <label class="trace-name plain" crop="end"/>
+                </hbox>
               </hbox>
-              <vbox class="devtools-separator"/>
-              <toolbarbutton id="toggle-breakpoints"
-                             class="devtools-toolbarbutton"
-                             tooltiptext="&debuggerUI.sources.toggleBreakpoints;"
-                             command="toggleBreakpointsCommand"/>
-            </toolbar>
-          </tabpanel>
-          <tabpanel id="callstack-tabpanel">
-            <vbox id="callstack-list" flex="1"/>
-          </tabpanel>
-          <tabpanel id="tracer-tabpanel">
-            <vbox id="tracer-traces" flex="1"/>
-            <hbox class="trace-item-template" hidden="true">
-              <hbox class="trace-item" align="center" flex="1" crop="end">
-                <label class="trace-type plain"/>
-                <label class="trace-name plain" crop="end"/>
-              </hbox>
-            </hbox>
-            <toolbar id="tracer-toolbar" class="devtools-toolbar">
-              <toolbarbutton id="clear-tracer"
-                             label="&debuggerUI.clearButton;"
-                             tooltiptext="&debuggerUI.clearButton.tooltip;"
-                             command="clearTraces"
-                             class="devtools-toolbarbutton"/>
-              <textbox id="tracer-search"
-                       class="devtools-searchinput"
-                       flex="1"
-                       type="search"/>
-            </toolbar>
-          </tabpanel>
-        </tabpanels>
-      </tabbox>
+              <toolbar id="tracer-toolbar" class="devtools-toolbar">
+                <toolbarbutton id="clear-tracer"
+                               label="&debuggerUI.clearButton;"
+                               tooltiptext="&debuggerUI.clearButton.tooltip;"
+                               command="clearTraces"
+                               class="devtools-toolbarbutton"/>
+                <textbox id="tracer-search"
+                         class="devtools-searchinput"
+                         flex="1"
+                         type="search"/>
+              </toolbar>
+            </tabpanel>
+          </tabpanels>
+        </tabbox>
+      </vbox>
       <splitter id="sources-and-editor-splitter"
                 class="devtools-side-splitter"/>
       <deck id="editor-deck" flex="1" class="devtools-main-content">
         <vbox id="editor"/>
         <vbox id="black-boxed-message"
               align="center"
               pack="center">
           <description id="black-boxed-message-label">

--- a/browser/devtools/debugger/test/browser_dbg_host-layout.js
+++ b/browser/devtools/debugger/test/browser_dbg_host-layout.js
@@ -66,23 +66,23 @@ function testHost(aTab, aPanel, aHostTyp
     "The default host type should've been set on the panel window (1).");
   is(gDebugger.gHostType, aHostType,
     "The default host type should've been set on the panel window (2).");
 
   is(gView._body.getAttribute("layout"), aLayoutType,
     "The default host type is present as an attribute on the panel's body.");
 
   if (aLayoutType == "horizontal") {
-    is(gView._sourcesPane.parentNode.id, "debugger-widgets",
-      "The sources pane's parent is correct for the horizontal layout.");
+    is(gView._workersAndSourcesPane.parentNode.id, "debugger-widgets",
+      "The workers and sources pane's parent is correct for the horizontal layout.");
     is(gView._instrumentsPane.parentNode.id, "debugger-widgets",
       "The instruments pane's parent is correct for the horizontal layout.");
   } else {
-    is(gView._sourcesPane.parentNode.id, "vertical-layout-panes-container",
-      "The sources pane's parent is correct for the vertical layout.");
+    is(gView._workersAndSourcesPane.parentNode.id, "vertical-layout-panes-container",
+      "The workers and sources pane's parent is correct for the vertical layout.");
     is(gView._instrumentsPane.parentNode.id, "vertical-layout-panes-container",
       "The instruments pane's parent is correct for the vertical layout.");
   }
 
   let widgets = gDebugger.document.getElementById("debugger-widgets").childNodes;
   let panes = gDebugger.document.getElementById("vertical-layout-panes-container").childNodes;
 
   if (aLayoutType == "horizontal") {

--- a/browser/devtools/debugger/test/browser_dbg_panel-size.js
+++ b/browser/devtools/debugger/test/browser_dbg_panel-size.js
@@ -12,71 +12,71 @@ function test() {
   let gTab, gPanel, gDebugger;
   let gPrefs, gSources, gInstruments;
 
   initDebugger(TAB_URL).then(([aTab,, aPanel]) => {
     gTab = aTab;
     gPanel = aPanel;
     gDebugger = gPanel.panelWin;
     gPrefs = gDebugger.Prefs;
-    gSources = gDebugger.document.getElementById("sources-pane");
+    gSources = gDebugger.document.getElementById("workers-and-sources-pane");
     gInstruments = gDebugger.document.getElementById("instruments-pane");
 
     waitForSourceShown(gPanel, ".html").then(performTest);
   });
 
   function performTest() {
-    let preferredSw = Services.prefs.getIntPref("devtools.debugger.ui.panes-sources-width");
+    let preferredWsw = Services.prefs.getIntPref("devtools.debugger.ui.panes-workers-and-sources-width");
     let preferredIw = Services.prefs.getIntPref("devtools.debugger.ui.panes-instruments-width");
     let someWidth1, someWidth2;
 
     do {
       someWidth1 = parseInt(Math.random() * 200) + 100;
       someWidth2 = parseInt(Math.random() * 300) + 100;
-    } while ((someWidth1 == preferredSw) || (someWidth2 == preferredIw));
+    } while ((someWidth1 == preferredWsw) || (someWidth2 == preferredIw));
 
-    info("Preferred sources width: " + preferredSw);
+    info("Preferred sources width: " + preferredWsw);
     info("Preferred instruments width: " + preferredIw);
     info("Generated sources width: " + someWidth1);
     info("Generated instruments width: " + someWidth2);
 
-    ok(gPrefs.sourcesWidth,
-      "The debugger preferences should have a saved sourcesWidth value.");
+    ok(gPrefs.workersAndSourcesWidth,
+      "The debugger preferences should have a saved workersAndSourcesWidth value.");
     ok(gPrefs.instrumentsWidth,
       "The debugger preferences should have a saved instrumentsWidth value.");
 
-    is(gPrefs.sourcesWidth, preferredSw,
-      "The debugger preferences should have a correct sourcesWidth value.");
+    is(gPrefs.workersAndSourcesWidth, preferredWsw,
+      "The debugger preferences should have a correct workersAndSourcesWidth value.");
     is(gPrefs.instrumentsWidth, preferredIw,
       "The debugger preferences should have a correct instrumentsWidth value.");
 
-    is(gSources.getAttribute("width"), gPrefs.sourcesWidth,
-      "The sources pane width should be the same as the preferred value.");
+    is(gSources.getAttribute("width"), gPrefs.workersAndSourcesWidth,
+      "The workers and sources pane width should be the same as the preferred value.");
     is(gInstruments.getAttribute("width"), gPrefs.instrumentsWidth,
       "The instruments pane width should be the same as the preferred value.");
 
     gSources.setAttribute("width", someWidth1);
     gInstruments.setAttribute("width", someWidth2);
 
-    is(gPrefs.sourcesWidth, preferredSw,
-      "The sources pane width pref should still be the same as the preferred value.");
+    is(gPrefs.workersAndSourcesWidth, preferredWsw,
+      "The workers and sources pane width pref should still be the same as the preferred value.");
     is(gPrefs.instrumentsWidth, preferredIw,
       "The instruments pane width pref should still be the same as the preferred value.");
 
-    isnot(gSources.getAttribute("width"), gPrefs.sourcesWidth,
-      "The sources pane width should not be the preferred value anymore.");
+    isnot(gSources.getAttribute("width"), gPrefs.workersAndSourcesWidth,
+      "The workers and sources pane width should not be the preferred value anymore.");
     isnot(gInstruments.getAttribute("width"), gPrefs.instrumentsWidth,
       "The instruments pane width should not be the preferred value anymore.");
 
     teardown(gPanel).then(() => {
-      is(gPrefs.sourcesWidth, someWidth1,
-        "The sources pane width should have been saved by now.");
+      is(gPrefs.workersAndSourcesWidth, someWidth1,
+        "The workers and sources pane width should have been saved by now.");
       is(gPrefs.instrumentsWidth, someWidth2,
         "The instruments pane width should have been saved by now.");
 
       // Cleanup after ourselves!
-      Services.prefs.setIntPref("devtools.debugger.ui.panes-sources-width", preferredSw);
+      Services.prefs.setIntPref("devtools.debugger.ui.panes-workers-and-sources-width", preferredWsw);
       Services.prefs.setIntPref("devtools.debugger.ui.panes-instruments-width", preferredIw);
 
       finish();
     });
   }
 }

new file mode 100644
--- /dev/null
+++ b/browser/devtools/debugger/views/workers-view.js
@@ -0,0 +1,36 @@
+/* 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/. */
+"use strict";
+
+function WorkersView() {}
+
+WorkersView.prototype = Heritage.extend(WidgetMethods, {
+  initialize: function () {
+    if (!Prefs.workersEnabled) {
+      return;
+    }
+
+    document.getElementById("workers-pane").removeAttribute("hidden");
+
+    this.widget = new SideMenuWidget(document.getElementById("workers"), {
+      showArrows: true,
+    });
+    this.emptyText = L10N.getStr("noWorkersText");
+  },
+
+  addWorker: function (actor, name) {
+    let element = document.createElement("label");
+    element.className = "plain dbg-worker-item";
+    element.setAttribute("value", name);
+    element.setAttribute("flex", "1");
+
+    this.push([element, actor], {});
+  },
+
+  removeWorker: function (actor) {
+    this.remove(this.getItemByValue(actor));
+  }
+});
+
+DebuggerView.Workers = new WorkersView();

--- a/browser/devtools/framework/toolbox-options.xul
+++ b/browser/devtools/framework/toolbox-options.xul
@@ -154,15 +154,20 @@
                       tooltiptext="&options.enableChrome.tooltip3;"
                       data-pref="devtools.chrome.enabled"/>
           </hbox>
           <hbox class="hidden-labels-box">
             <checkbox label="&options.enableRemote.label3;"
                       tooltiptext="&options.enableRemote.tooltip;"
                       data-pref="devtools.debugger.remote-enabled"/>
           </hbox>
+          <hbox class="hidden-labels-box">
+            <checkbox label="&options.enableWorkers.label;"
+                      tooltiptext="&options.enableWorkers.tooltip;"
+                      data-pref="devtools.debugger.workers"/>
+          </hbox>
           <label class="options-citation-label theme-comment"
           >&options.context.triggersPageRefresh;</label>
         </vbox>
       </vbox>
     </hbox>
   </hbox>
 </window>

--- a/browser/devtools/jar.mn
+++ b/browser/devtools/jar.mn
@@ -63,16 +63,17 @@ browser.jar:
     content/browser/devtools/codemirror/foldgutter.js                  (sourceeditor/codemirror/fold/foldgutter.js)
     content/browser/devtools/codemirror/tern.js                        (sourceeditor/codemirror/tern/tern.js)
     content/browser/devtools/codemirror/show-hint.js                   (sourceeditor/codemirror/hint/show-hint.js)
     content/browser/devtools/codemirror/mozilla.css                    (sourceeditor/codemirror/mozilla.css)
     content/browser/devtools/debugger.xul                              (debugger/debugger.xul)
     content/browser/devtools/debugger.css                              (debugger/debugger.css)
     content/browser/devtools/debugger-controller.js                    (debugger/debugger-controller.js)
     content/browser/devtools/debugger-view.js                          (debugger/debugger-view.js)
+    content/browser/devtools/debugger/workers-view.js                  (debugger/views/workers-view.js)
     content/browser/devtools/debugger/sources-view.js                  (debugger/views/sources-view.js)
     content/browser/devtools/debugger/variable-bubble-view.js          (debugger/views/variable-bubble-view.js)
     content/browser/devtools/debugger/tracer-view.js                   (debugger/views/tracer-view.js)
     content/browser/devtools/debugger/watch-expressions-view.js        (debugger/views/watch-expressions-view.js)
     content/browser/devtools/debugger/event-listeners-view.js          (debugger/views/event-listeners-view.js)
     content/browser/devtools/debugger/global-search-view.js            (debugger/views/global-search-view.js)
     content/browser/devtools/debugger/toolbar-view.js                  (debugger/views/toolbar-view.js)
     content/browser/devtools/debugger/options-view.js                  (debugger/views/options-view.js)

--- a/browser/locales/en-US/chrome/browser/devtools/debugger.dtd
+++ b/browser/locales/en-US/chrome/browser/devtools/debugger.dtd
@@ -160,16 +160,17 @@
 <!-- LOCALIZATION NOTE (debuggerUI.seMenuCondBreak): This is the text that
   -  appears in the source editor context menu for adding a conditional
   -  breakpoint. -->
 <!ENTITY debuggerUI.seMenuCondBreak     "Add Conditional Breakpoint">
 <!ENTITY debuggerUI.seMenuCondBreak.key "B">
 
 <!-- LOCALIZATION NOTE (debuggerUI.tabs.*): This is the text that
   -  appears in the debugger's side pane tabs. -->
+<!ENTITY debuggerUI.tabs.workers        "Workers">
 <!ENTITY debuggerUI.tabs.sources        "Sources">
 <!ENTITY debuggerUI.tabs.traces         "Traces">
 <!ENTITY debuggerUI.tabs.callstack      "Call Stack">
 <!ENTITY debuggerUI.tabs.variables      "Variables">
 <!ENTITY debuggerUI.tabs.events         "Events">
 
 <!-- LOCALIZATION NOTE (debuggerUI.seMenuAddWatch): This is the text that
   -  appears in the source editor context menu for adding an expression. -->

--- a/browser/locales/en-US/chrome/browser/devtools/debugger.properties
+++ b/browser/locales/en-US/chrome/browser/devtools/debugger.properties
@@ -73,18 +73,22 @@ stepInTooltip=Step In (%S)
 # LOCALIZATION NOTE (stepOutTooltip): The label that is displayed on the
 # button that steps out of a function call.
 stepOutTooltip=Step Out (%S)
 
 # LOCALIZATION NOTE (emptyGlobalsText): The text to display in the menulist
 # when there are no chrome globals available.
 noGlobalsText=No globals
 
-# LOCALIZATION NOTE (noSourcesText): The text to display in the sources menu
-# when there are no scripts.
+# LOCALIZATION NOTE (noWorkersText): The text to display in the workers list
+# when there are no workers.
+noWorkersText=This page has no workers.
+
+# LOCALIZATION NOTE (noSourcesText): The text to display in the sources list
+# when there are no sources.
 noSourcesText=This page has no sources.
 
 # LOCALIZATION NOTE (loadingSourcesText): The text to display in the sources menu
 # when waiting for scripts to load.
 loadingSourcesText=Waiting for sources…
 
 # LOCALIZATION NOTE (noEventListenersText): The text to display in the events tab
 # when there are no events.
@@ -315,9 +319,10 @@ functionSearchSeparatorLabel=←
 # open in separate tabs and the user tries to resume them in the wrong order.
 # The substitution parameter is the URL of the last paused window that must be
 # resumed first.
 resumptionOrderPanelTitle=There are one or more paused debuggers. Please resume the most-recently paused debugger first at: %S
 
 variablesViewOptimizedOut=(optimized away)
 variablesViewUninitialized=(uninitialized)
 variablesViewMissingArgs=(unavailable)
+
 anonymousSourcesLabel=Anonymous Sources

--- a/browser/locales/en-US/chrome/browser/devtools/toolbox.dtd
+++ b/browser/locales/en-US/chrome/browser/devtools/toolbox.dtd
@@ -87,16 +87,22 @@
 <!ENTITY options.enableChrome.tooltip3  "Turning this option on will allow you to use various developer tools in browser context (via Tools > Web Developer > Browser Toolbox) and debug add-ons from the Add-ons Manager">
 
 <!-- LOCALIZATION NOTE (options.enableRemote.label3): This is the label for the
   -  checkbox that toggles remote debugging, i.e. devtools.debugger.remote-enabled
   -  boolean preference in about:config, in the options panel. -->
 <!ENTITY options.enableRemote.label3    "Enable remote debugging">
 <!ENTITY options.enableRemote.tooltip   "Turning this option on will allow the developer tools to debug remote Firefox instance like Firefox OS">
 
+<!-- LOCALIZATION NOTE (options.enableWorkers.label): This is the label for the
+  -  checkbox that toggles worker debugging, i.e. devtools.debugger.workers
+  -  boolean preference in about:config, in the options panel. -->
+<!ENTITY options.enableWorkers.label    "Enable worker debugging (in development)">
+<!ENTITY options.enableWorkers.tooltip  "Turning this option on will allow the developer tools to debug workers">
+
 <!-- LOCALIZATION NOTE (options.disableJavaScript.label,
   -  options.disableJavaScript.tooltip): This is the options panel label and
   -  tooltip for the checkbox that toggles JavaScript on or off. -->
 <!ENTITY options.disableJavaScript.label     "Disable JavaScript *">
 <!ENTITY options.disableJavaScript.tooltip   "Turning this option on will disable JavaScript for the current tab. If the tab or the toolbox is closed then this setting will be forgotten.">
 
 <!-- LOCALIZATION NOTE (options.disableCache.label2,
   -  options.disableCache.tooltip2): This is the options panel label and

--- a/toolkit/devtools/client/dbg-client.jsm
+++ b/toolkit/devtools/client/dbg-client.jsm
@@ -1359,16 +1359,17 @@ TabClient.prototype = {
 
 eventSource(TabClient.prototype);
 
 function WorkerClient(aClient, aForm) {
   this.client = aClient;
   this._actor = aForm.from;
   this._isClosed = false;
   this._isFrozen = aForm.isFrozen;
+  this._url = aForm.url;
 
   this._onClose = this._onClose.bind(this);
   this._onFreeze = this._onFreeze.bind(this);
   this._onThaw = this._onThaw.bind(this);
 
   this.addListener("close", this._onClose);
   this.addListener("freeze", this._onFreeze);
   this.addListener("thaw", this._onThaw);
@@ -1382,16 +1383,20 @@ WorkerClient.prototype = {
   get request() {
     return this.client.request;
   },
 
   get actor() {
     return this._actor;
   },
 
+  get url() {
+    return this._url;
+  },
+
   get isClosed() {
     return this._isClosed;
   },
 
   get isFrozen() {
     return this._isFrozen;
   },
 

--- a/toolkit/devtools/server/actors/webbrowser.js
+++ b/toolkit/devtools/server/actors/webbrowser.js
@@ -1086,16 +1086,17 @@ TabActor.prototype = {
   onListFrames: function BTA_onListFrames(aRequest) {
     let windows = this._docShellsToWindows(this.docShells);
     return { frames: windows };
   },
 
   onListWorkers: function BTA_onListWorkers(aRequest) {
     if (this._workerActorList === null) {
       this._workerActorList = new WorkerActorList({
+        type: Ci.nsIWorkerDebugger.TYPE_DEDICATED,
         window: this.window
       });
     }
 
     return this._workerActorList.getList().then((actors) => {
       let pool = new ActorPool(this.conn);
       for (let actor of actors) {
         pool.addActor(actor);

--- a/toolkit/devtools/server/actors/worker.js
+++ b/toolkit/devtools/server/actors/worker.js
@@ -7,16 +7,19 @@ Cu.import("resource://gre/modules/XPCOMU
 
 XPCOMUtils.defineLazyServiceGetter(
   this, "wdm",
   "@mozilla.org/dom/workers/workerdebuggermanager;1",
   "nsIWorkerDebuggerManager"
 );
 
 function matchWorkerDebugger(dbg, options) {
+  if ("type" in options && dbg.type !== options.type) {
+    return false;
+  }
   if ("window" in options) {
     let window = dbg.window;
     while (window !== null && window.parent !== window) {
       window = window.parent;
     }
 
     if (window !== options.window) {
       return false;
@@ -50,17 +53,18 @@ WorkerActor.prototype = {
 
     if (!this._isAttached) {
       this._dbg.addListener(this);
       this._isAttached = true;
     }
 
     return {
       type: "attached",
-      isFrozen: this._dbg.isFrozen
+      isFrozen: this._dbg.isFrozen,
+      url: this._dbg.url
     };
   },
 
   onDetach: function () {
     if (!this._isAttached) {
       return { error: "wrongState" };
     }