--- a/dom/system/gonk/systemlibs.js
+++ b/dom/system/gonk/systemlibs.js
@@ -1,16 +1,19 @@
 /* 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/. */
 
 const SYSTEM_PROPERTY_KEY_MAX = 32;
 const SYSTEM_PROPERTY_VALUE_MAX = 92;
 
-const DEBUG=false;
+// We leave this as 'undefined' instead of setting it to 'false'. That
+// way a file that includes us can have it defined already without us
+// overriding the value here.
+let DEBUG;
 
 /**
  * Expose some system-level functions.
  */
 let libcutils = (function() {
   let lib;
   try {
     lib = ctypes.open("libcutils.so");

--- a/dom/wifi/WifiWorker.js
+++ b/dom/wifi/WifiWorker.js
@@ -5,17 +5,17 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 const {classes: Cc, interfaces: Ci, utils: Cu, results: Cr} = Components;
 
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
-const DEBUG = false; // set to false to suppress debug messages
+const DEBUG = false; // set to true to show debug messages
 
 const WIFIWORKER_CONTRACTID = "@mozilla.org/wifi/worker;1";
 const WIFIWORKER_CID        = Components.ID("{a14e8977-d259-433a-a88d-58dd44657e5b}");
 
 const WIFIWORKER_WORKER     = "resource://gre/modules/wifi_worker.js";
 
 // A note about errors and error handling in this file:
 // The libraries that we use in this file are intended for C code. For

--- a/dom/wifi/wifi_worker.js
+++ b/dom/wifi/wifi_worker.js
@@ -8,17 +8,17 @@
 
 importScripts("libhardware_legacy.js", "systemlibs.js");
 
 var cbuf = ctypes.char.array(4096)();
 var hwaddr = ctypes.uint8_t.array(6)();
 var len = ctypes.size_t();
 var ints = ctypes.int.array(8)();
 
-const DEBUG = false;
+let DEBUG = false;
 
 let debug;
 if (DEBUG) {
   debug = function (s) {
     dump("-*- WifiWorker component: " + s + "\n");
   };
 } else {
   debug = function (s) {};