--- a/config/config.mk
+++ b/config/config.mk
@@ -24,16 +24,91 @@ topsrcdir	= $(DEPTH)
 endif
 
 ifndef INCLUDED_AUTOCONF_MK
 include $(DEPTH)/config/autoconf.mk
 endif
 
 -include $(DEPTH)/.mozconfig.mk
 
+# Integrate with mozbuild-generated make files. We first verify that no
+# variables provided by the automatically generated .mk files are
+# present. If they are, this is a violation of the separation of
+# responsibility between Makefile.in and mozbuild files.
+_MOZBUILD_EXTERNAL_VARIABLES := \
+  ANDROID_GENERATED_RESFILES \
+  ANDROID_RESFILES \
+  CMMSRCS \
+  CPP_UNIT_TESTS \
+  DIRS \
+  EXTRA_PP_COMPONENTS \
+  EXTRA_PP_JS_MODULES \
+  GTEST_CMMSRCS \
+  GTEST_CPPSRCS \
+  GTEST_CSRCS \
+  HOST_CSRCS \
+  HOST_LIBRARY_NAME \
+  IS_COMPONENT \
+  JS_MODULES_PATH \
+  LIBRARY_NAME \
+  LIBXUL_LIBRARY \
+  MODULE \
+  MSVC_ENABLE_PGO \
+  NO_DIST_INSTALL \
+  PARALLEL_DIRS \
+  SDK_HEADERS \
+  SIMPLE_PROGRAMS \
+  TEST_DIRS \
+  TIERS \
+  TOOL_DIRS \
+  XPCSHELL_TESTS \
+  XPIDL_MODULE \
+  $(NULL)
+
+_DEPRECATED_VARIABLES := \
+  MOCHITEST_FILES_PARTS \
+  MOCHITEST_BROWSER_FILES_PARTS \
+  $(NULL)
+
+ifndef EXTERNALLY_MANAGED_MAKE_FILE
+# Using $(firstword) may not be perfect. But it should be good enough for most
+# scenarios.
+_current_makefile = $(CURDIR)/$(firstword $(MAKEFILE_LIST))
+
+$(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
+    $(error Variable $(var) is defined in $(_current_makefile). It should only be defined in moz.build files),\
+    ))
+
+$(foreach var,$(_DEPRECATED_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
+    $(error Variable $(var) is defined in $(_current_makefile). This variable has been deprecated. It does nothing. It must be removed in order to build)\
+    ))
+
+# Import the automatically generated backend file. If this file doesn't exist,
+# the backend hasn't been properly configured. We want this to be a fatal
+# error, hence not using "-include".
+ifndef STANDALONE_MAKEFILE
+GLOBAL_DEPS += backend.mk
+include backend.mk
+endif
+
+# Freeze the values specified by moz.build to catch them if they fail.
+
+$(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES),$(eval $(var)_FROZEN := '$($(var))'))
+$(foreach var,$(_DEPRECATED_VARIABLES),$(eval $(var)_FROZEN := '$($(var))'))
+
+CHECK_MOZBUILD_VARIABLES = $(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES), \
+  $(if $(subst $($(var)_FROZEN),,'$($(var))'), \
+	  $(error Variable $(var) is defined in $(_current_makefile). It should only be defined in moz.build files),\
+  )) $(foreach var,$(_DEPRECATED_VARIABLES), \
+	$(if $(subst $($(var)_FROZEN),,'$($(var))'), \
+    $(error Variable $(var) is defined in $(_current_makefile). This variable has been deprecated. It does nothing. It must be removed in order to build),\
+    ))
+
+endif
+
 space = $(NULL) $(NULL)
 
 # Include defs.mk files that can be found in $(srcdir)/$(DEPTH),
 # $(srcdir)/$(DEPTH-1), $(srcdir)/$(DEPTH-2), etc., and $(srcdir)
 # where $(DEPTH-1) is one level less of depth, $(DEPTH-2), two, etc.
 # i.e. for DEPTH=../../.., DEPTH-1 is ../.. and DEPTH-2 is ..
 # These defs.mk files are used to define variables in a directory
 # and all its subdirectories, recursively.

--- a/config/rules.mk
+++ b/config/rules.mk
@@ -11,82 +11,19 @@ ifndef topsrcdir
 endif
 
 # Define an include-at-most-once flag
 ifdef INCLUDED_RULES_MK
 $(error Do not include rules.mk twice!)
 endif
 INCLUDED_RULES_MK = 1
 
-# Integrate with mozbuild-generated make files. We first verify that no
-# variables provided by the automatically generated .mk files are
-# present. If they are, this is a violation of the separation of
-# responsibility between Makefile.in and mozbuild files.
-_MOZBUILD_EXTERNAL_VARIABLES := \
-  ANDROID_GENERATED_RESFILES \
-  ANDROID_RESFILES \
-  CMMSRCS \
-  CPP_UNIT_TESTS \
-  DIRS \
-  EXTRA_PP_COMPONENTS \
-  EXTRA_PP_JS_MODULES \
-  GTEST_CMMSRCS \
-  GTEST_CPPSRCS \
-  GTEST_CSRCS \
-  HOST_CSRCS \
-  HOST_LIBRARY_NAME \
-  IS_COMPONENT \
-  JS_MODULES_PATH \
-  LIBRARY_NAME \
-  LIBXUL_LIBRARY \
-  MODULE \
-  MSVC_ENABLE_PGO \
-  NO_DIST_INSTALL \
-  PARALLEL_DIRS \
-  SDK_HEADERS \
-  SIMPLE_PROGRAMS \
-  TEST_DIRS \
-  TIERS \
-  TOOL_DIRS \
-  XPCSHELL_TESTS \
-  XPIDL_MODULE \
-  $(NULL)
-
-_DEPRECATED_VARIABLES := \
-  XPIDL_FLAGS \
-  MOCHITEST_FILES_PARTS \
-  MOCHITEST_BROWSER_FILES_PARTS \
-  MOCHITEST_WEBAPPRT_CHROME_FILES \
-  $(NULL)
-
-ifndef EXTERNALLY_MANAGED_MAKE_FILE
-# Using $(firstword) may not be perfect. But it should be good enough for most
-# scenarios.
-_current_makefile = $(CURDIR)/$(firstword $(MAKEFILE_LIST))
-
-$(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
-    $(error Variable $(var) is defined in $(_current_makefile). It should only be defined in moz.build files),\
-    ))
-
-$(foreach var,$(_DEPRECATED_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
-    $(error Variable $(var) is defined in $(_current_makefile). This variable has been deprecated. It does nothing. It must be removed in order to build)\
-    ))
-
-ifneq (,$(XPIDLSRCS)$(SDK_XPIDLSRCS))
-    $(error XPIDLSRCS and SDK_XPIDLSRCS have been merged and moved to moz.build files as the XPIDL_SOURCES variable. You must move these variables out of $(_current_makefile))
-endif
-
-# Import the automatically generated backend file. If this file doesn't exist,
-# the backend hasn't been properly configured. We want this to be a fatal
-# error, hence not using "-include".
-ifndef STANDALONE_MAKEFILE
-GLOBAL_DEPS += backend.mk
-include backend.mk
-endif
-endif
+# Make sure that anything that needs to be defined in moz.build wasn't
+# overwritten.
+_eval_for_side_effects := $(CHECK_MOZBUILD_VARIABLES)
 
 ifndef MOZILLA_DIR
 MOZILLA_DIR = $(topsrcdir)
 endif
 
 ifndef INCLUDED_CONFIG_MK
 include $(topsrcdir)/config/config.mk
 endif

--- a/config/tests/src-simple/Makefile.in
+++ b/config/tests/src-simple/Makefile.in
@@ -1,18 +1,18 @@
 #
 # 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/.
 
 LOCALE_SRCDIR = $(srcdir)/l10n
 
-include $(topsrcdir)/config/config.mk
+STANDALONE_MAKEFILE := 1
 
-STANDALONE_MAKEFILE := 1
+include $(topsrcdir)/config/config.mk
 
 XPI_NAME = test_jar_mn
 
 DEFINES += \
 	-DAB_CD=ab-X-stuff \
 	$(NULL)
 
 MY_MANIFEST = $(if $(USE_EXTENSION_MANIFEST), $(FINAL_TARGET)/chrome.manifest, $(FINAL_TARGET)/chrome/test.manifest)

--- a/js/src/config/config.mk
+++ b/js/src/config/config.mk
@@ -24,16 +24,91 @@ topsrcdir	= $(DEPTH)
 endif
 
 ifndef INCLUDED_AUTOCONF_MK
 include $(DEPTH)/config/autoconf.mk
 endif
 
 -include $(DEPTH)/.mozconfig.mk
 
+# Integrate with mozbuild-generated make files. We first verify that no
+# variables provided by the automatically generated .mk files are
+# present. If they are, this is a violation of the separation of
+# responsibility between Makefile.in and mozbuild files.
+_MOZBUILD_EXTERNAL_VARIABLES := \
+  ANDROID_GENERATED_RESFILES \
+  ANDROID_RESFILES \
+  CMMSRCS \
+  CPP_UNIT_TESTS \
+  DIRS \
+  EXTRA_PP_COMPONENTS \
+  EXTRA_PP_JS_MODULES \
+  GTEST_CMMSRCS \
+  GTEST_CPPSRCS \
+  GTEST_CSRCS \
+  HOST_CSRCS \
+  HOST_LIBRARY_NAME \
+  IS_COMPONENT \
+  JS_MODULES_PATH \
+  LIBRARY_NAME \
+  LIBXUL_LIBRARY \
+  MODULE \
+  MSVC_ENABLE_PGO \
+  NO_DIST_INSTALL \
+  PARALLEL_DIRS \
+  SDK_HEADERS \
+  SIMPLE_PROGRAMS \
+  TEST_DIRS \
+  TIERS \
+  TOOL_DIRS \
+  XPCSHELL_TESTS \
+  XPIDL_MODULE \
+  $(NULL)
+
+_DEPRECATED_VARIABLES := \
+  MOCHITEST_FILES_PARTS \
+  MOCHITEST_BROWSER_FILES_PARTS \
+  $(NULL)
+
+ifndef EXTERNALLY_MANAGED_MAKE_FILE
+# Using $(firstword) may not be perfect. But it should be good enough for most
+# scenarios.
+_current_makefile = $(CURDIR)/$(firstword $(MAKEFILE_LIST))
+
+$(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
+    $(error Variable $(var) is defined in $(_current_makefile). It should only be defined in moz.build files),\
+    ))
+
+$(foreach var,$(_DEPRECATED_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
+    $(error Variable $(var) is defined in $(_current_makefile). This variable has been deprecated. It does nothing. It must be removed in order to build)\
+    ))
+
+# Import the automatically generated backend file. If this file doesn't exist,
+# the backend hasn't been properly configured. We want this to be a fatal
+# error, hence not using "-include".
+ifndef STANDALONE_MAKEFILE
+GLOBAL_DEPS += backend.mk
+include backend.mk
+endif
+
+# Freeze the values specified by moz.build to catch them if they fail.
+
+$(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES),$(eval $(var)_FROZEN := '$($(var))'))
+$(foreach var,$(_DEPRECATED_VARIABLES),$(eval $(var)_FROZEN := '$($(var))'))
+
+CHECK_MOZBUILD_VARIABLES = $(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES), \
+  $(if $(subst $($(var)_FROZEN),,'$($(var))'), \
+	  $(error Variable $(var) is defined in $(_current_makefile). It should only be defined in moz.build files),\
+  )) $(foreach var,$(_DEPRECATED_VARIABLES), \
+	$(if $(subst $($(var)_FROZEN),,'$($(var))'), \
+    $(error Variable $(var) is defined in $(_current_makefile). This variable has been deprecated. It does nothing. It must be removed in order to build),\
+    ))
+
+endif
+
 space = $(NULL) $(NULL)
 
 # Include defs.mk files that can be found in $(srcdir)/$(DEPTH),
 # $(srcdir)/$(DEPTH-1), $(srcdir)/$(DEPTH-2), etc., and $(srcdir)
 # where $(DEPTH-1) is one level less of depth, $(DEPTH-2), two, etc.
 # i.e. for DEPTH=../../.., DEPTH-1 is ../.. and DEPTH-2 is ..
 # These defs.mk files are used to define variables in a directory
 # and all its subdirectories, recursively.

--- a/js/src/config/rules.mk
+++ b/js/src/config/rules.mk
@@ -11,82 +11,19 @@ ifndef topsrcdir
 endif
 
 # Define an include-at-most-once flag
 ifdef INCLUDED_RULES_MK
 $(error Do not include rules.mk twice!)
 endif
 INCLUDED_RULES_MK = 1
 
-# Integrate with mozbuild-generated make files. We first verify that no
-# variables provided by the automatically generated .mk files are
-# present. If they are, this is a violation of the separation of
-# responsibility between Makefile.in and mozbuild files.
-_MOZBUILD_EXTERNAL_VARIABLES := \
-  ANDROID_GENERATED_RESFILES \
-  ANDROID_RESFILES \
-  CMMSRCS \
-  CPP_UNIT_TESTS \
-  DIRS \
-  EXTRA_PP_COMPONENTS \
-  EXTRA_PP_JS_MODULES \
-  GTEST_CMMSRCS \
-  GTEST_CPPSRCS \
-  GTEST_CSRCS \
-  HOST_CSRCS \
-  HOST_LIBRARY_NAME \
-  IS_COMPONENT \
-  JS_MODULES_PATH \
-  LIBRARY_NAME \
-  LIBXUL_LIBRARY \
-  MODULE \
-  MSVC_ENABLE_PGO \
-  NO_DIST_INSTALL \
-  PARALLEL_DIRS \
-  SDK_HEADERS \
-  SIMPLE_PROGRAMS \
-  TEST_DIRS \
-  TIERS \
-  TOOL_DIRS \
-  XPCSHELL_TESTS \
-  XPIDL_MODULE \
-  $(NULL)
-
-_DEPRECATED_VARIABLES := \
-  XPIDL_FLAGS \
-  MOCHITEST_FILES_PARTS \
-  MOCHITEST_BROWSER_FILES_PARTS \
-  MOCHITEST_WEBAPPRT_CHROME_FILES \
-  $(NULL)
-
-ifndef EXTERNALLY_MANAGED_MAKE_FILE
-# Using $(firstword) may not be perfect. But it should be good enough for most
-# scenarios.
-_current_makefile = $(CURDIR)/$(firstword $(MAKEFILE_LIST))
-
-$(foreach var,$(_MOZBUILD_EXTERNAL_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
-    $(error Variable $(var) is defined in $(_current_makefile). It should only be defined in moz.build files),\
-    ))
-
-$(foreach var,$(_DEPRECATED_VARIABLES),$(if $(filter file override,$(subst $(NULL) ,_,$(origin $(var)))),\
-    $(error Variable $(var) is defined in $(_current_makefile). This variable has been deprecated. It does nothing. It must be removed in order to build)\
-    ))
-
-ifneq (,$(XPIDLSRCS)$(SDK_XPIDLSRCS))
-    $(error XPIDLSRCS and SDK_XPIDLSRCS have been merged and moved to moz.build files as the XPIDL_SOURCES variable. You must move these variables out of $(_current_makefile))
-endif
-
-# Import the automatically generated backend file. If this file doesn't exist,
-# the backend hasn't been properly configured. We want this to be a fatal
-# error, hence not using "-include".
-ifndef STANDALONE_MAKEFILE
-GLOBAL_DEPS += backend.mk
-include backend.mk
-endif
-endif
+# Make sure that anything that needs to be defined in moz.build wasn't
+# overwritten.
+_eval_for_side_effects := $(CHECK_MOZBUILD_VARIABLES)
 
 ifndef MOZILLA_DIR
 MOZILLA_DIR = $(topsrcdir)
 endif
 
 ifndef INCLUDED_CONFIG_MK
 include $(topsrcdir)/config/config.mk
 endif