diff --git a/extensions/pref/autoconfig/src/Makefile.in b/extensions/pref/autoconfig/src/Makefile.in
index 830f5ae62ab..4f6b6673204 100644
--- a/extensions/pref/autoconfig/src/Makefile.in
+++ b/extensions/pref/autoconfig/src/Makefile.in
@@ -5,3 +5,17 @@
 AUTOCFG_JS_EXPORTS = \
 		$(srcdir)/prefcalls.js \
 		$(NULL)
+
+ifeq (cocoa,$(MOZ_WIDGET_TOOLKIT))
+AUTOCFG_JS_EXPORTS += $(srcdir)/mac/platform.js
+else
+ifeq ($(MOZ_WIDGET_TOOLKIT), windows)
+AUTOCFG_JS_EXPORTS += $(srcdir)/win/platform.js
+else
+ifeq ($(MOZ_WIDGET_TOOLKIT), beos)
+AUTOCFG_JS_EXPORTS += $(srcdir)/beos/platform.js
+else
+AUTOCFG_JS_EXPORTS += $(srcdir)/unix/platform.js
+endif
+endif
+endif
diff --git a/extensions/pref/autoconfig/src/beos/platform.js b/extensions/pref/autoconfig/src/beos/platform.js
new file mode 100644
index 00000000000..1292a963f9b
--- /dev/null
+++ b/extensions/pref/autoconfig/src/beos/platform.js
@@ -0,0 +1,6 @@
+/* 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/. */
+
+// BeOS specific auto configuration preference defaults
+platform.value = "beos";
diff --git a/extensions/pref/autoconfig/src/mac/platform.js b/extensions/pref/autoconfig/src/mac/platform.js
new file mode 100644
index 00000000000..1852f4c33aa
--- /dev/null
+++ b/extensions/pref/autoconfig/src/mac/platform.js
@@ -0,0 +1,6 @@
+/* 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/. */
+
+// Mac specific auto configuration preference defaults
+platform.value = "macintosh";
diff --git a/extensions/pref/autoconfig/src/nsReadConfig.cpp b/extensions/pref/autoconfig/src/nsReadConfig.cpp
index d995baf1b3f..85a0c148ad7 100644
--- a/extensions/pref/autoconfig/src/nsReadConfig.cpp
+++ b/extensions/pref/autoconfig/src/nsReadConfig.cpp
@@ -158,6 +158,11 @@ nsresult nsReadConfig::readConfigFile()
         if (NS_FAILED(rv)) 
             return rv;
 
+        // Evaluate platform specific directives
+        rv = openAndEvaluateJSFile("platform.js", 0, false, false);
+        if (NS_FAILED(rv)) 
+            return rv;
+
         mRead = true;
     }
     // If the lockFileName is nullptr return ok, because no lockFile will be used
diff --git a/extensions/pref/autoconfig/src/prefcalls.js b/extensions/pref/autoconfig/src/prefcalls.js
index 723f771b0fc..751584bcb7c 100644
--- a/extensions/pref/autoconfig/src/prefcalls.js
+++ b/extensions/pref/autoconfig/src/prefcalls.js
@@ -10,6 +10,9 @@ const LDAPSyncQueryContractID = "@mozilla.org/ldapsyncquery;1";
 const nsIPrefService = Components.interfaces.nsIPrefService;
 const PrefServiceContractID = "@mozilla.org/preferences-service;1";
 
+// set on a platform specific basis in platform.js
+platform = { value: "" };
+
 var gVersion;
 
 function getPrefBranch() {
diff --git a/extensions/pref/autoconfig/src/unix/platform.js b/extensions/pref/autoconfig/src/unix/platform.js
new file mode 100644
index 00000000000..2489346b740
--- /dev/null
+++ b/extensions/pref/autoconfig/src/unix/platform.js
@@ -0,0 +1,6 @@
+/* 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/. */
+
+// Unix specific auto configuration preference defaults
+platform.value = "unix";
diff --git a/extensions/pref/autoconfig/src/win/platform.js b/extensions/pref/autoconfig/src/win/platform.js
new file mode 100644
index 00000000000..e7077f73f6e
--- /dev/null
+++ b/extensions/pref/autoconfig/src/win/platform.js
@@ -0,0 +1,6 @@
+/* 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/. */
+
+// Windows specific auto configuration preference defaults
+platform.value = "windows";