--- a/devtools/client/webconsole/webconsole-connection-proxy.js
+++ b/devtools/client/webconsole/webconsole-connection-proxy.js
@@ -173,16 +173,21 @@ WebConsoleConnectionProxy.prototype = {
 
   /**
    * Attach to the Web Console actor.
    * @private
    */
   _attachConsole: function () {
     let listeners = ["PageError", "ConsoleAPI", "NetworkActivity",
                      "FileActivity"];
+    // Enable the forwarding of console messages to the parent process
+    // when we open the Browser Console or Toolbox.
+    if (this.target.chrome && !this.target.isAddon) {
+      listeners.push("ContentProcessMessages");
+    }
     this.client.attachConsole(this._consoleActor, listeners,
                               this._onAttachConsole);
   },
 
   /**
    * The "attachConsole" response handler.
    *
    * @private

--- a/devtools/server/actors/webconsole.js
+++ b/devtools/server/actors/webconsole.js
@@ -34,16 +34,17 @@ loader.lazyRequireGetter(this, "Environm
 // to load an unsupported module.
 if (isWorker) {
   loader.lazyRequireGetter(this, "ConsoleAPIListener", "devtools/server/actors/webconsole/worker-listeners", true);
   loader.lazyRequireGetter(this, "ConsoleServiceListener", "devtools/server/actors/webconsole/worker-listeners", true);
 } else {
   loader.lazyRequireGetter(this, "ConsoleAPIListener", "devtools/server/actors/webconsole/listeners", true);
   loader.lazyRequireGetter(this, "ConsoleServiceListener", "devtools/server/actors/webconsole/listeners", true);
   loader.lazyRequireGetter(this, "ConsoleReflowListener", "devtools/server/actors/webconsole/listeners", true);
+  loader.lazyRequireGetter(this, "ContentProcessListener", "devtools/server/actors/webconsole/listeners", true);
 }
 
 /**
  * The WebConsoleActor implements capabilities needed for the Web Console
  * feature.
  *
  * @constructor
  * @param object connection
@@ -363,16 +364,20 @@ WebConsoleActor.prototype =
     if (this.consoleReflowListener) {
       this.consoleReflowListener.destroy();
       this.consoleReflowListener = null;
     }
     if (this.serverLoggingListener) {
       this.serverLoggingListener.destroy();
       this.serverLoggingListener = null;
     }
+    if (this.contentProcessListener) {
+      this.contentProcessListener.destroy();
+      this.contentProcessListener = null;
+    }
 
     events.off(this.parentActor, "changed-toplevel-document",
                this._onChangedToplevelDocument);
 
     this.conn.removeActorPool(this._actorPool);
 
     if (this.parentActor.isRootActor) {
       Services.obs.removeObserver(this._onObserverNotification,
@@ -659,16 +664,26 @@ WebConsoleActor.prototype =
             break;
           }
           if (!this.serverLoggingListener) {
             this.serverLoggingListener =
               new ServerLoggingListener(this.window, this);
           }
           startedListeners.push(listener);
           break;
+        case "ContentProcessMessages":
+          // Workers don't support this message type
+          if (isWorker) {
+            break;
+          }
+          if (!this.contentProcessListener) {
+            this.contentProcessListener = new ContentProcessListener(this);
+          }
+          startedListeners.push(listener);
+          break;
       }
     }
 
     // Update the live list of running listeners
     startedListeners.forEach(this._listeners.add, this._listeners);
 
     return {
       startedListeners: startedListeners,
@@ -688,17 +703,17 @@ WebConsoleActor.prototype =
    */
   onStopListeners: function (request) {
     let stoppedListeners = [];
 
     // If no specific listeners are requested to be detached, we stop all
     // listeners.
     let toDetach = request.listeners ||
       ["PageError", "ConsoleAPI", "NetworkActivity",
-       "FileActivity", "ServerLogging"];
+       "FileActivity", "ServerLogging", "ContentProcessMessages"];
 
     while (toDetach.length > 0) {
       let listener = toDetach.shift();
       switch (listener) {
         case "PageError":
           if (this.consoleServiceListener) {
             this.consoleServiceListener.destroy();
             this.consoleServiceListener = null;
@@ -744,16 +759,23 @@ WebConsoleActor.prototype =
           break;
         case "ServerLogging":
           if (this.serverLoggingListener) {
             this.serverLoggingListener.destroy();
             this.serverLoggingListener = null;
           }
           stoppedListeners.push(listener);
           break;
+        case "ContentProcessMessages":
+          if (this.contentProcessListener) {
+            this.contentProcessListener.destroy();
+            this.contentProcessListener = null;
+          }
+          stoppedListeners.push(listener);
+          break;
       }
     }
 
     // Update the live list of running listeners
     stoppedListeners.forEach(this._listeners.delete, this._listeners);
 
     return { stoppedListeners: stoppedListeners };
   },

copy from toolkit/components/processsingleton/ContentProcessSingleton.js
copy to devtools/server/actors/webconsole/content-process-forward.js
--- a/toolkit/components/processsingleton/ContentProcessSingleton.js
+++ b/devtools/server/actors/webconsole/content-process-forward.js
@@ -2,25 +2,22 @@
  * 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";
 
 const Cu = Components.utils;
 const Ci = Components.interfaces;
 
-Cu.import("resource://gre/modules/Services.jsm");
-Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+const { Services } = Cu.import("resource://gre/modules/Services.jsm", {});
+const { XPCOMUtils } = Cu.import("resource://gre/modules/XPCOMUtils.jsm", {});
 
 XPCOMUtils.defineLazyServiceGetter(this, "cpmm",
                                    "@mozilla.org/childprocessmessagemanager;1",
                                    "nsIMessageSender");
-
-XPCOMUtils.defineLazyModuleGetter(this, "TelemetryController",
-                                  "resource://gre/modules/TelemetryController.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "E10SUtils",
                                   "resource:///modules/E10SUtils.jsm");
 
 /*
  * The message manager has an upper limit on message sizes that it can
  * reliably forward to the parent so we limit the size of console log event
  * messages that we forward here. The web console is local and receives the
  * full console message, but addons subscribed to console event messages
@@ -35,92 +32,105 @@ XPCOMUtils.defineLazyModuleGetter(this, 
  * MSG_MGR_CONSOLE_INFO_MAX characters. We don't attempt to calculate
  * the exact amount of space the message manager implementation will require
  * for a given message so this is imperfect.
  */
 const MSG_MGR_CONSOLE_MAX_SIZE = 1024 * 1024; // 1MB
 const MSG_MGR_CONSOLE_VAR_SIZE = 8;
 const MSG_MGR_CONSOLE_INFO_MAX = 1024;
 
-function ContentProcessSingleton() {}
-ContentProcessSingleton.prototype = {
-  classID: Components.ID("{ca2a8470-45c7-11e4-916c-0800200c9a66}"),
+function ContentProcessForward() {
+  Services.obs.addObserver(this, "console-api-log-event");
+  Services.obs.addObserver(this, "xpcom-shutdown");
+  cpmm.addMessageListener("DevTools:StopForwardingContentProcessMessage", this);
+}
+ContentProcessForward.prototype = {
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver,
                                          Ci.nsISupportsWeakReference]),
 
+  receiveMessage(message) {
+    if (message.name == "DevTools:StopForwardingContentProcessMessage") {
+      this.uninit();
+    }
+  },
+
   observe(subject, topic, data) {
     switch (topic) {
-    case "app-startup": {
-      Services.obs.addObserver(this, "console-api-log-event");
-      Services.obs.addObserver(this, "xpcom-shutdown");
-      TelemetryController.observe(null, topic, null);
-      break;
-    }
-    case "console-api-log-event": {
-      let consoleMsg = subject.wrappedJSObject;
+      case "console-api-log-event": {
+        let consoleMsg = subject.wrappedJSObject;
 
-      let msgData = {
-        level: consoleMsg.level,
-        filename: consoleMsg.filename.substring(0, MSG_MGR_CONSOLE_INFO_MAX),
-        lineNumber: consoleMsg.lineNumber,
-        functionName: consoleMsg.functionName &&
-          consoleMsg.functionName.substring(0, MSG_MGR_CONSOLE_INFO_MAX),
-        timeStamp: consoleMsg.timeStamp,
-        addonId: consoleMsg.addonId,
-        arguments: [],
-      };
+        let msgData = {
+          level: consoleMsg.level,
+          filename: consoleMsg.filename.substring(0, MSG_MGR_CONSOLE_INFO_MAX),
+          lineNumber: consoleMsg.lineNumber,
+          functionName: consoleMsg.functionName &&
+            consoleMsg.functionName.substring(0, MSG_MGR_CONSOLE_INFO_MAX),
+          timeStamp: consoleMsg.timeStamp,
+          addonId: consoleMsg.addonId,
+          arguments: [],
+        };
+
+        // We can't send objects over the message manager, so we sanitize
+        // them out, replacing those arguments with "<unavailable>".
+        let unavailString = "<unavailable>";
+        let unavailStringLength = unavailString.length * 2; // 2-bytes per char
 
-      // We can't send objects over the message manager, so we sanitize
-      // them out, replacing those arguments with "<unavailable>".
-      let unavailString = "<unavailable>";
-      let unavailStringLength = unavailString.length * 2; // 2-bytes per char
-
-      // When the sum of argument sizes reaches MSG_MGR_CONSOLE_MAX_SIZE,
-      // replace all arguments with "<truncated>".
-      let totalArgLength = 0;
+        // When the sum of argument sizes reaches MSG_MGR_CONSOLE_MAX_SIZE,
+        // replace all arguments with "<truncated>".
+        let totalArgLength = 0;
 
-      // Walk through the arguments, checking the type and size.
-      for (let arg of consoleMsg.arguments) {
-        if ((typeof arg == "object" || typeof arg == "function") &&
-            arg !== null) {
-          if (Services.appinfo.remoteType === E10SUtils.EXTENSION_REMOTE_TYPE) {
-            // For OOP extensions: we want the developer to be able to see the
-            // logs in the Browser Console. When the Addon Toolbox will be more
-            // prominent we can revisit.
-            try {
-              // If the argument is clonable, then send it as-is. If
-              // cloning fails, fall back to the unavailable string.
-              arg = Cu.cloneInto(arg, {});
-            } catch (e) {
+        // Walk through the arguments, checking the type and size.
+        for (let arg of consoleMsg.arguments) {
+          if ((typeof arg == "object" || typeof arg == "function") &&
+              arg !== null) {
+            if (Services.appinfo.remoteType === E10SUtils.EXTENSION_REMOTE_TYPE) {
+              // For OOP extensions: we want the developer to be able to see the
+              // logs in the Browser Console. When the Addon Toolbox will be more
+              // prominent we can revisit.
+              try {
+                // If the argument is clonable, then send it as-is. If
+                // cloning fails, fall back to the unavailable string.
+                arg = Cu.cloneInto(arg, {});
+              } catch (e) {
+                arg = unavailString;
+              }
+            } else {
               arg = unavailString;
             }
+            totalArgLength += unavailStringLength;
+          } else if (typeof arg == "string") {
+            totalArgLength += arg.length * 2; // 2-bytes per char
           } else {
-            arg = unavailString;
+            totalArgLength += MSG_MGR_CONSOLE_VAR_SIZE;
           }
-          totalArgLength += unavailStringLength;
-        } else if (typeof arg == "string") {
-          totalArgLength += arg.length * 2; // 2-bytes per char
-        } else {
-          totalArgLength += MSG_MGR_CONSOLE_VAR_SIZE;
+
+          if (totalArgLength <= MSG_MGR_CONSOLE_MAX_SIZE) {
+            msgData.arguments.push(arg);
+          } else {
+            // arguments take up too much space
+            msgData.arguments = ["<truncated>"];
+            break;
+          }
         }
 
-        if (totalArgLength <= MSG_MGR_CONSOLE_MAX_SIZE) {
-          msgData.arguments.push(arg);
-        } else {
-          // arguments take up too much space
-          msgData.arguments = ["<truncated>"];
-          break;
-        }
+        cpmm.sendAsyncMessage("Console:Log", msgData);
+        break;
       }
 
-      cpmm.sendAsyncMessage("Console:Log", msgData);
-      break;
-    }
-
-    case "xpcom-shutdown":
-      Services.obs.removeObserver(this, "console-api-log-event");
-      Services.obs.removeObserver(this, "xpcom-shutdown");
-      break;
+      case "xpcom-shutdown":
+        this.uninit();
+        break;
     }
   },
+
+  uninit() {
+    Services.obs.removeObserver(this, "console-api-log-event");
+    Services.obs.removeObserver(this, "xpcom-shutdown");
+    cpmm.removeMessageListener("DevTools:StopForwardingContentProcessMessage", this);
+  }
 };
 
-this.NSGetFactory = XPCOMUtils.generateNSGetFactory([ContentProcessSingleton]);
+// loadProcessScript loads in all processes, including the parent,
+// in which we don't need any forwarding
+if (Services.appinfo.processType == Services.appinfo.PROCESS_TYPE_CONTENT) {
+  new ContentProcessForward();
+}
+

--- a/devtools/server/actors/webconsole/listeners.js
+++ b/devtools/server/actors/webconsole/listeners.js
@@ -10,16 +10,19 @@ const Services = require("Services");
 const {XPCOMUtils} = require("resource://gre/modules/XPCOMUtils.jsm");
 const {CONSOLE_WORKER_IDS, WebConsoleUtils} = require("devtools/server/actors/webconsole/utils");
 
 XPCOMUtils.defineLazyServiceGetter(this,
                                    "swm",
                                    "@mozilla.org/serviceworkers/manager;1",
                                    "nsIServiceWorkerManager");
 
+// Process script used to forward console calls from content processes to parent process
+const CONTENT_PROCESS_SCRIPT = "resource://devtools/server/actors/webconsole/content-process-forward.js";
+
 // The page errors listener
 
 /**
  * The nsIConsoleService listener. This is used to send all of the console
  * messages (JavaScript, CSS and more) to the remote Web Console instance.
  *
  * @constructor
  * @param nsIDOMWindow [window]
@@ -446,8 +449,44 @@ ConsoleReflowListener.prototype =
   /**
    * Unregister listener.
    */
   destroy: function () {
     this.docshell.removeWeakReflowObserver(this);
     this.listener = this.docshell = null;
   },
 };
+
+/**
+ * Forward console message calls from content processes to the parent process.
+ * Used by Browser console and toolbox to see messages from all processes.
+ *
+ * @constructor
+ * @param object owner
+ *        The listener owner which needs to implement:
+ *        - onConsoleAPICall(message)
+ */
+function ContentProcessListener(listener) {
+  this.listener = listener;
+
+  Services.ppmm.addMessageListener("Console:Log", this);
+  Services.ppmm.loadProcessScript(CONTENT_PROCESS_SCRIPT, true);
+}
+
+exports.ContentProcessListener = ContentProcessListener;
+
+ContentProcessListener.prototype = {
+  receiveMessage(message) {
+    let logMsg = message.data;
+    logMsg.wrappedJSObject = logMsg;
+    this.listener.onConsoleAPICall(logMsg);
+  },
+
+  destroy() {
+    // Tell the content processes to stop listening and forwarding messages
+    Services.ppmm.broadcastAsyncMessage("DevTools:StopForwardingContentProcessMessage");
+
+    Services.ppmm.removeMessageListener("Console:Log", this);
+    Services.ppmm.removeDelayedProcessScript(CONTENT_PROCESS_SCRIPT);
+
+    this.listener = null;
+  }
+};

--- a/devtools/server/actors/webconsole/moz.build
+++ b/devtools/server/actors/webconsole/moz.build
@@ -1,11 +1,12 @@
 # -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
 # 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/.
 
 DevToolsModules(
+    'content-process-forward.js',
     'listeners.js',
     'utils.js',
     'worker-listeners.js',
 )

--- a/toolkit/components/processsingleton/ContentProcessSingleton.js
+++ b/toolkit/components/processsingleton/ContentProcessSingleton.js
@@ -5,122 +5,32 @@
 "use strict";
 
 const Cu = Components.utils;
 const Ci = Components.interfaces;
 
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
-XPCOMUtils.defineLazyServiceGetter(this, "cpmm",
-                                   "@mozilla.org/childprocessmessagemanager;1",
-                                   "nsIMessageSender");
-
 XPCOMUtils.defineLazyModuleGetter(this, "TelemetryController",
                                   "resource://gre/modules/TelemetryController.jsm");
-XPCOMUtils.defineLazyModuleGetter(this, "E10SUtils",
-                                  "resource:///modules/E10SUtils.jsm");
-
-/*
- * The message manager has an upper limit on message sizes that it can
- * reliably forward to the parent so we limit the size of console log event
- * messages that we forward here. The web console is local and receives the
- * full console message, but addons subscribed to console event messages
- * in the parent receive the truncated version. Due to fragmentation,
- * messages as small as 1MB have resulted in IPC allocation failures on
- * 32-bit platforms. To limit IPC allocation sizes, console.log messages
- * with arguments with total size > MSG_MGR_CONSOLE_MAX_SIZE (bytes) have
- * their arguments completely truncated. MSG_MGR_CONSOLE_VAR_SIZE is an
- * approximation of how much space (in bytes) a JS non-string variable will
- * require in the manager's implementation. For strings, we use 2 bytes per
- * char. The console message URI and function name are limited to
- * MSG_MGR_CONSOLE_INFO_MAX characters. We don't attempt to calculate
- * the exact amount of space the message manager implementation will require
- * for a given message so this is imperfect.
- */
-const MSG_MGR_CONSOLE_MAX_SIZE = 1024 * 1024; // 1MB
-const MSG_MGR_CONSOLE_VAR_SIZE = 8;
-const MSG_MGR_CONSOLE_INFO_MAX = 1024;
 
 function ContentProcessSingleton() {}
 ContentProcessSingleton.prototype = {
   classID: Components.ID("{ca2a8470-45c7-11e4-916c-0800200c9a66}"),
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver,
                                          Ci.nsISupportsWeakReference]),
 
   observe(subject, topic, data) {
     switch (topic) {
     case "app-startup": {
-      Services.obs.addObserver(this, "console-api-log-event");
       Services.obs.addObserver(this, "xpcom-shutdown");
       TelemetryController.observe(null, topic, null);
       break;
     }
-    case "console-api-log-event": {
-      let consoleMsg = subject.wrappedJSObject;
-
-      let msgData = {
-        level: consoleMsg.level,
-        filename: consoleMsg.filename.substring(0, MSG_MGR_CONSOLE_INFO_MAX),
-        lineNumber: consoleMsg.lineNumber,
-        functionName: consoleMsg.functionName &&
-          consoleMsg.functionName.substring(0, MSG_MGR_CONSOLE_INFO_MAX),
-        timeStamp: consoleMsg.timeStamp,
-        addonId: consoleMsg.addonId,
-        arguments: [],
-      };
-
-      // We can't send objects over the message manager, so we sanitize
-      // them out, replacing those arguments with "<unavailable>".
-      let unavailString = "<unavailable>";
-      let unavailStringLength = unavailString.length * 2; // 2-bytes per char
-
-      // When the sum of argument sizes reaches MSG_MGR_CONSOLE_MAX_SIZE,
-      // replace all arguments with "<truncated>".
-      let totalArgLength = 0;
-
-      // Walk through the arguments, checking the type and size.
-      for (let arg of consoleMsg.arguments) {
-        if ((typeof arg == "object" || typeof arg == "function") &&
-            arg !== null) {
-          if (Services.appinfo.remoteType === E10SUtils.EXTENSION_REMOTE_TYPE) {
-            // For OOP extensions: we want the developer to be able to see the
-            // logs in the Browser Console. When the Addon Toolbox will be more
-            // prominent we can revisit.
-            try {
-              // If the argument is clonable, then send it as-is. If
-              // cloning fails, fall back to the unavailable string.
-              arg = Cu.cloneInto(arg, {});
-            } catch (e) {
-              arg = unavailString;
-            }
-          } else {
-            arg = unavailString;
-          }
-          totalArgLength += unavailStringLength;
-        } else if (typeof arg == "string") {
-          totalArgLength += arg.length * 2; // 2-bytes per char
-        } else {
-          totalArgLength += MSG_MGR_CONSOLE_VAR_SIZE;
-        }
-
-        if (totalArgLength <= MSG_MGR_CONSOLE_MAX_SIZE) {
-          msgData.arguments.push(arg);
-        } else {
-          // arguments take up too much space
-          msgData.arguments = ["<truncated>"];
-          break;
-        }
-      }
-
-      cpmm.sendAsyncMessage("Console:Log", msgData);
-      break;
-    }
-
     case "xpcom-shutdown":
-      Services.obs.removeObserver(this, "console-api-log-event");
       Services.obs.removeObserver(this, "xpcom-shutdown");
       break;
     }
   },
 };
 
 this.NSGetFactory = XPCOMUtils.generateNSGetFactory([ContentProcessSingleton]);

--- a/toolkit/components/processsingleton/MainProcessSingleton.js
+++ b/toolkit/components/processsingleton/MainProcessSingleton.js
@@ -13,22 +13,16 @@ XPCOMUtils.defineLazyModuleGetter(this, 
                                   "resource://gre/modules/NetUtil.jsm");
 
 function MainProcessSingleton() {}
 MainProcessSingleton.prototype = {
   classID: Components.ID("{0636a680-45cb-11e4-916c-0800200c9a66}"),
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver,
                                          Ci.nsISupportsWeakReference]),
 
-  logConsoleMessage(message) {
-    let logMsg = message.data;
-    logMsg.wrappedJSObject = logMsg;
-    Services.obs.notifyObservers(logMsg, "console-api-log-event");
-  },
-
   // Called when a webpage calls window.external.AddSearchProvider
   addSearchEngine({ target: browser, data: { pageURL, engineURL } }) {
     pageURL = NetUtil.newURI(pageURL);
     engineURL = NetUtil.newURI(engineURL, null, pageURL);
 
     let iconURL;
     let tabbrowser = browser.getTabBrowser();
     if (browser.mIconURL && (!tabbrowser || tabbrowser.shouldLoadFavIcon(pageURL)))
@@ -68,23 +62,21 @@ MainProcessSingleton.prototype = {
     switch (topic) {
     case "app-startup": {
       Services.obs.addObserver(this, "xpcom-shutdown");
 
       // Load this script early so that console.* is initialized
       // before other frame scripts.
       Services.mm.loadFrameScript("chrome://global/content/browser-content.js", true);
       Services.ppmm.loadProcessScript("chrome://global/content/process-content.js", true);
-      Services.ppmm.addMessageListener("Console:Log", this.logConsoleMessage);
       Services.mm.addMessageListener("Search:AddEngine", this.addSearchEngine);
       Services.ppmm.loadProcessScript("resource:///modules/ContentObservers.js", true);
       break;
     }
 
     case "xpcom-shutdown":
-      Services.ppmm.removeMessageListener("Console:Log", this.logConsoleMessage);
       Services.mm.removeMessageListener("Search:AddEngine", this.addSearchEngine);
       break;
     }
   },
 };
 
 this.NSGetFactory = XPCOMUtils.generateNSGetFactory([MainProcessSingleton]);