--- a/accessible/atk/moz.build
+++ b/accessible/atk/moz.build
@@ -50,10 +50,8 @@ if CONFIG['MOZ_ENABLE_GTK']:
 if CONFIG['MOZ_ENABLE_DBUS']:
     CXXFLAGS += CONFIG['MOZ_DBUS_CFLAGS']
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 if CONFIG['CLANG_CXX']:
     # Suppress clang warning about unused function from gobject's RTTI macros.
     CXXFLAGS += ['-Wno-unused-function']
-
-FAIL_ON_WARNINGS = True

--- a/accessible/base/moz.build
+++ b/accessible/base/moz.build
@@ -92,10 +92,8 @@ else:
     ]
 
 FINAL_LIBRARY = 'xul'
 
 if CONFIG['MOZ_ENABLE_GTK']:
     CXXFLAGS += CONFIG['MOZ_CAIRO_CFLAGS']
 
 include('/ipc/chromium/chromium-config.mozbuild')
-
-FAIL_ON_WARNINGS = True

--- a/accessible/generic/moz.build
+++ b/accessible/generic/moz.build
@@ -52,10 +52,8 @@ elif CONFIG['MOZ_WIDGET_TOOLKIT'] == 'co
 else:
     LOCAL_INCLUDES += [
         '/accessible/other',
     ]
 
 FINAL_LIBRARY = 'xul'
 
 include('/ipc/chromium/chromium-config.mozbuild')
-
-FAIL_ON_WARNINGS = True

--- a/accessible/html/moz.build
+++ b/accessible/html/moz.build
@@ -38,10 +38,8 @@ elif CONFIG['MOZ_WIDGET_TOOLKIT'] == 'co
         '/accessible/mac',
     ]
 else:
     LOCAL_INCLUDES += [
         '/accessible/other',
     ]
 
 FINAL_LIBRARY = 'xul'
-
-FAIL_ON_WARNINGS = True

--- a/accessible/ipc/moz.build
+++ b/accessible/ipc/moz.build
@@ -42,10 +42,8 @@ if CONFIG['ACCESSIBILITY']:
     else:
         LOCAL_INCLUDES += [
             '/accessible/other',
         ]
 
     FINAL_LIBRARY = 'xul'
 
 include('/ipc/chromium/chromium-config.mozbuild')
-
-FAIL_ON_WARNINGS = True

--- a/accessible/mac/moz.build
+++ b/accessible/mac/moz.build
@@ -36,10 +36,8 @@ LOCAL_INCLUDES += [
     '/layout/xul',
     '/widget',
     '/widget/cocoa',
 ]
 
 FINAL_LIBRARY = 'xul'
 
 include('/ipc/chromium/chromium-config.mozbuild')
-
-FAIL_ON_WARNINGS = True

--- a/accessible/other/moz.build
+++ b/accessible/other/moz.build
@@ -17,10 +17,8 @@ SOURCES += [
 LOCAL_INCLUDES += [
     '/accessible/base',
     '/accessible/generic',
     '/accessible/html',
     '/accessible/xul',
 ]
 
 FINAL_LIBRARY = 'xul'
-
-FAIL_ON_WARNINGS = True

--- a/accessible/windows/ia2/moz.build
+++ b/accessible/windows/ia2/moz.build
@@ -45,11 +45,9 @@ LOCAL_INCLUDES += [
     '/accessible/xul',
 ]
 
 FINAL_LIBRARY = 'xul'
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
 
-FAIL_ON_WARNINGS = True
-
 include('/ipc/chromium/chromium-config.mozbuild')

--- a/accessible/windows/msaa/moz.build
+++ b/accessible/windows/msaa/moz.build
@@ -59,10 +59,8 @@ LOCAL_INCLUDES += [
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
-
-FAIL_ON_WARNINGS = True

--- a/accessible/windows/sdn/moz.build
+++ b/accessible/windows/sdn/moz.build
@@ -18,10 +18,8 @@ LOCAL_INCLUDES += [
     '/accessible/xpcom',
     '/accessible/xul',
 ]
 
 FINAL_LIBRARY = 'xul'
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
-
-FAIL_ON_WARNINGS = True

--- a/accessible/windows/uia/moz.build
+++ b/accessible/windows/uia/moz.build
@@ -13,10 +13,8 @@ LOCAL_INCLUDES += [
     '/accessible/generic',
     '/accessible/html',
     '/accessible/windows/msaa',
     '/accessible/xpcom',
     '/accessible/xul',
 ]
 
 FINAL_LIBRARY = 'xul'
-
-FAIL_ON_WARNINGS = True

--- a/accessible/xpcom/moz.build
+++ b/accessible/xpcom/moz.build
@@ -43,10 +43,10 @@ elif CONFIG['MOZ_WIDGET_TOOLKIT'] == 'co
     ]
 else:
     LOCAL_INCLUDES += [
         '/accessible/other',
     ]
 
 FINAL_LIBRARY = 'xul'
 
-if CONFIG['GNU_CXX']:
-    FAIL_ON_WARNINGS = True
+if not CONFIG['GNU_CXX']:
+    ALLOW_COMPILER_WARNINGS = True

--- a/accessible/xul/moz.build
+++ b/accessible/xul/moz.build
@@ -43,10 +43,8 @@ elif CONFIG['MOZ_WIDGET_TOOLKIT'] == 'co
         '/accessible/mac',
     ]
 else:
     LOCAL_INCLUDES += [
         '/accessible/other',
     ]
 
 FINAL_LIBRARY = 'xul'
-
-FAIL_ON_WARNINGS = True

--- a/b2g/app/moz.build
+++ b/b2g/app/moz.build
@@ -73,17 +73,15 @@ DISABLE_STL_WRAPPING = True
 if CONFIG['OS_ARCH'] == 'WINNT':
     OS_LIBS += [
         'version',
     ]
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
 
-FAIL_ON_WARNINGS = True
-
 JS_PREFERENCE_FILES += [
     'b2g.js',
 ]
 
 DIST_FILES += [
   'ua-update.json.in',
 ]

--- a/b2g/config/mozconfigs/linux32_gecko/debug
+++ b/b2g/config/mozconfigs/linux32_gecko/debug
@@ -14,17 +14,17 @@ ac_add_options --enable-debug
 # by 2 MBs.
 STRIP_FLAGS="--strip-debug"
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Use sccache
 no_sccache=
 
 #B2G options
 ac_add_options --enable-application=b2g
 ENABLE_MARIONETTE=1

--- a/b2g/config/mozconfigs/linux32_gecko/nightly
+++ b/b2g/config/mozconfigs/linux32_gecko/nightly
@@ -14,17 +14,17 @@ ac_add_options --enable-signmar
 # by 2 MBs.
 STRIP_FLAGS="--strip-debug"
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Use sccache
 no_sccache=
 
 #B2G options
 ac_add_options --enable-application=b2g
 export CXXFLAGS=-DMOZ_ENABLE_JS_DUMP

--- a/b2g/config/mozconfigs/linux64_gecko/debug
+++ b/b2g/config/mozconfigs/linux64_gecko/debug
@@ -14,17 +14,17 @@ ac_add_options --enable-debug
 # by 2 MBs.
 STRIP_FLAGS="--strip-debug"
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Use sccache
 no_sccache=
 
 #B2G options
 ac_add_options --enable-application=b2g
 ENABLE_MARIONETTE=1

--- a/b2g/config/mozconfigs/linux64_gecko/nightly
+++ b/b2g/config/mozconfigs/linux64_gecko/nightly
@@ -14,17 +14,17 @@ ac_add_options --enable-signmar
 # by 2 MBs.
 STRIP_FLAGS="--strip-debug"
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Use sccache
 no_sccache=
 
 #B2G options
 ac_add_options --enable-application=b2g
 export CXXFLAGS=-DMOZ_ENABLE_JS_DUMP

--- a/b2g/config/mozconfigs/macosx64_gecko/debug
+++ b/b2g/config/mozconfigs/macosx64_gecko/debug
@@ -13,17 +13,17 @@ ac_add_options --enable-signmar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 #ac_add_options --with-macbundlename-prefix=Firefox
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # B2G Stuff
 ac_add_options --enable-application=b2g
 ac_add_options --enable-debug-symbols
 ac_add_options --enable-debug
 ENABLE_MARIONETTE=1
 

--- a/b2g/config/mozconfigs/macosx64_gecko/nightly
+++ b/b2g/config/mozconfigs/macosx64_gecko/nightly
@@ -14,17 +14,17 @@ ac_add_options --enable-signmar
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 #ac_add_options --with-macbundlename-prefix=Firefox
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # B2G Stuff
 ac_add_options --enable-application=b2g
 ac_add_options --enable-debug-symbols
 export CXXFLAGS=-DMOZ_ENABLE_JS_DUMP
 
 GAIADIR=$topsrcdir/gaia

--- a/b2g/dev/config/mozconfigs/macosx64/mulet
+++ b/b2g/dev/config/mozconfigs/macosx64/mulet
@@ -12,17 +12,17 @@ ac_add_options --enable-profiling
 ac_add_options --enable-instruments
 ac_add_options --enable-dtrace
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 ac_add_options --with-macbundlename-prefix=Firefox
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
 
 # Include Firefox OS fonts.
 MOZTTDIR=$topsrcdir/moz-tt
 

--- a/b2g/gaia/moz.build
+++ b/b2g/gaia/moz.build
@@ -15,10 +15,8 @@ if not CONFIG['MOZ_B2GDROID']:
         DEFINES['B2G_NAME'] = 'L"%s-bin%s"' % (PROGRAM, CONFIG['BIN_SUFFIX'])
         DEFINES['GAIA_PATH'] = 'L"gaia\\\\profile"'
     else:
         SOURCES += [
             'run-b2g.c',
         ]
         DEFINES['B2G_NAME'] = '"%s-bin%s"' % (PROGRAM, CONFIG['BIN_SUFFIX'])
         DEFINES['GAIA_PATH'] = '"gaia/profile"'
-
-FAIL_ON_WARNINGS = True

--- a/browser/app/moz.build
+++ b/browser/app/moz.build
@@ -81,10 +81,8 @@ if CONFIG['MOZ_LINKER']:
 
 if CONFIG['HAVE_CLOCK_MONOTONIC']:
     OS_LIBS += CONFIG['REALTIME_LIBS']
 
 JAR_MANIFESTS += ['jar.mn']
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
-
-FAIL_ON_WARNINGS = True

--- a/browser/components/about/moz.build
+++ b/browser/components/about/moz.build
@@ -12,10 +12,8 @@ SOURCES += [
     'AboutRedirector.cpp',
 ]
 
 FINAL_LIBRARY = 'browsercomps'
 
 LOCAL_INCLUDES += [
     '../build',
 ]
-
-FAIL_ON_WARNINGS = True

--- a/browser/components/build/moz.build
+++ b/browser/components/build/moz.build
@@ -29,10 +29,8 @@ if CONFIG['OS_ARCH'] == 'WINNT':
         'shlwapi',
         'version',
     ]
 
 # Mac: Need to link with CoreFoundation for Mac Migrators (PList reading code)
 # GTK2: Need to link with glib for GNOME shell service
 if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('cocoa', 'gtk2', 'gtk3'):
     OS_LIBS += CONFIG['TK_LIBS']
-
-FAIL_ON_WARNINGS = True

--- a/browser/components/dirprovider/moz.build
+++ b/browser/components/dirprovider/moz.build
@@ -16,10 +16,8 @@ XPCSHELL_TESTS_MANIFESTS += [
     'tests/unit/xpcshell.ini',
 ]
 
 FINAL_LIBRARY = 'browsercomps'
 
 LOCAL_INCLUDES += [
     '../build'
 ]
-
-FAIL_ON_WARNINGS = True

--- a/browser/components/feeds/moz.build
+++ b/browser/components/feeds/moz.build
@@ -35,12 +35,10 @@ FINAL_LIBRARY = 'browsercomps'
 
 for var in ('MOZ_APP_NAME', 'MOZ_MACBUNDLE_NAME'):
     DEFINES[var] = CONFIG[var]
 
 LOCAL_INCLUDES += [
     '../build',
 ]
 
-FAIL_ON_WARNINGS = True
-
 with Files('**'):
     BUG_COMPONENT = ('Firefox', 'RSS Discovery and Preview')

--- a/browser/components/migration/moz.build
+++ b/browser/components/migration/moz.build
@@ -57,12 +57,10 @@ EXTRA_PP_JS_MODULES += [
 
 if CONFIG['OS_ARCH'] == 'WINNT':
     EXTRA_JS_MODULES += [
         'MSMigrationUtils.jsm',
     ]
 
 FINAL_LIBRARY = 'browsercomps'
 
-FAIL_ON_WARNINGS = True
-
 with Files('**'):
     BUG_COMPONENT = ('Firefox', 'Migration')

--- a/browser/components/shell/moz.build
+++ b/browser/components/shell/moz.build
@@ -45,12 +45,10 @@ EXTRA_COMPONENTS += [
     'nsSetDefaultBrowser.manifest',
 ]
 
 for var in ('MOZ_APP_NAME', 'MOZ_APP_VERSION'):
     DEFINES[var] = '"%s"' % CONFIG[var]
 
 CXXFLAGS += CONFIG['TK_CFLAGS']
 
-FAIL_ON_WARNINGS = True
-
 with Files('**'):
     BUG_COMPONENT = ('Firefox', 'Shell Integration')

--- a/browser/config/mozconfigs/linux32/common-opt
+++ b/browser/config/mozconfigs/linux32/common-opt
@@ -8,13 +8,13 @@ ac_add_options --with-mozilla-api-keyfil
 
 . $topsrcdir/build/unix/mozconfig.linux32
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1

--- a/browser/config/mozconfigs/linux32/debug
+++ b/browser/config/mozconfigs/linux32/debug
@@ -7,17 +7,17 @@ MOZ_AUTOMATION_L10N_CHECK=0
 
 . $topsrcdir/build/unix/mozconfig.linux32
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 #Use ccache
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
 
 ac_add_options --with-branding=browser/branding/nightly
 
 . "$topsrcdir/build/mozconfig.common.override"

--- a/browser/config/mozconfigs/linux64/common-opt
+++ b/browser/config/mozconfigs/linux64/common-opt
@@ -8,13 +8,13 @@ ac_add_options --with-mozilla-api-keyfil
 
 . $topsrcdir/build/unix/mozconfig.linux
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1

--- a/browser/config/mozconfigs/linux64/debug
+++ b/browser/config/mozconfigs/linux64/debug
@@ -5,17 +5,17 @@ ac_add_options --with-google-oauth-api-k
 
 MOZ_AUTOMATION_L10N_CHECK=0
 
 . $topsrcdir/build/unix/mozconfig.linux
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
 
 ac_add_options --with-branding=browser/branding/nightly
 
 . "$topsrcdir/build/mozconfig.common.override"

--- a/browser/config/mozconfigs/macosx-universal/common-opt
+++ b/browser/config/mozconfigs/macosx-universal/common-opt
@@ -11,13 +11,13 @@ ac_add_options --with-google-api-keyfile
 ac_add_options --with-google-oauth-api-keyfile=/builds/google-oauth-api.key
 ac_add_options --with-mozilla-api-keyfile=/builds/mozilla-desktop-geoloc-api.key
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1

--- a/browser/config/mozconfigs/macosx64/debug
+++ b/browser/config/mozconfigs/macosx64/debug
@@ -7,17 +7,17 @@ ac_add_options --with-google-oauth-api-k
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 if test "${MOZ_UPDATE_CHANNEL}" = "nightly"; then
 ac_add_options --with-macbundlename-prefix=Firefox
 fi
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
 
 ac_add_options --with-branding=browser/branding/nightly
 
 . "$topsrcdir/build/macosx/mozconfig.rust"

--- a/browser/config/mozconfigs/macosx64/debug-static-analysis
+++ b/browser/config/mozconfigs/macosx64/debug-static-analysis
@@ -2,15 +2,15 @@ MOZ_AUTOMATION_BUILD_SYMBOLS=0
 MOZ_AUTOMATION_PACKAGE_TESTS=0
 MOZ_AUTOMATION_L10N_CHECK=0
 
 . $topsrcdir/build/macosx/mozconfig.common
 
 ac_add_options --enable-debug
 ac_add_options --enable-dmd
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 ac_add_options --enable-clang-plugin
 
 . "$topsrcdir/build/mozconfig.common.override"
 . "$topsrcdir/build/mozconfig.cache"

--- a/browser/config/mozconfigs/macosx64/nightly
+++ b/browser/config/mozconfigs/macosx64/nightly
@@ -5,17 +5,17 @@ ac_add_options --with-google-oauth-api-k
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 if test "${MOZ_UPDATE_CHANNEL}" = "nightly"; then
 ac_add_options --with-macbundlename-prefix=Firefox
 fi
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
 
 ac_add_options --with-branding=browser/branding/nightly
 
 . "$topsrcdir/build/macosx/mozconfig.rust"

--- a/browser/config/mozconfigs/win32/common-opt
+++ b/browser/config/mozconfigs/win32/common-opt
@@ -24,17 +24,17 @@ ac_add_options --with-mozilla-api-keyfil
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
 . $topsrcdir/build/win32/mozconfig.vs2013-win64
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Enable Adobe Primetime CDM on 32-bit Windows in Mozilla builds.
 # Enabled here on the assumption that downstream vendors will not be using
 # these build configs.
 ac_add_options --enable-eme=adobe
 
 # Package js shell.

--- a/browser/config/mozconfigs/win32/debug
+++ b/browser/config/mozconfigs/win32/debug
@@ -15,17 +15,17 @@ else
 fi
 ac_add_options --with-google-oauth-api-keyfile=${_google_oauth_api_keyfile}
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 . $topsrcdir/build/win32/mozconfig.vs2013-win64
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
 
 ac_add_options --with-branding=browser/branding/nightly
 
 . "$topsrcdir/build/mozconfig.common.override"

--- a/browser/config/mozconfigs/win64/common-opt
+++ b/browser/config/mozconfigs/win64/common-opt
@@ -20,15 +20,15 @@ fi
 ac_add_options --with-google-oauth-api-keyfile=${_google_oauth_api_keyfile}
 ac_add_options --with-mozilla-api-keyfile=/c/builds/mozilla-desktop-geoloc-api.key
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
 export MOZ_TELEMETRY_REPORTING=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 . $topsrcdir/build/win64/mozconfig.vs2013
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1

--- a/browser/config/mozconfigs/win64/debug
+++ b/browser/config/mozconfigs/win64/debug
@@ -15,17 +15,17 @@ if [ -f /c/builds/google-oauth-api.key ]
 else
   _google_oauth_api_keyfile=/e/builds/google-oauth-api.key
 fi
 ac_add_options --with-google-oauth-api-keyfile=${_google_oauth_api_keyfile}
 
 # Needed to enable breakpad in application.ini
 export MOZILLA_OFFICIAL=1
 
-# Treat warnings as errors in directories with FAIL_ON_WARNINGS.
+# Treat warnings as errors (modulo ALLOW_COMPILER_WARNINGS).
 ac_add_options --enable-warnings-as-errors
 
 # Package js shell.
 export MOZ_PACKAGE_JSSHELL=1
 
 ac_add_options --with-branding=browser/branding/nightly
 
 . $topsrcdir/build/win64/mozconfig.vs2013

--- a/build/stlport/moz.build
+++ b/build/stlport/moz.build
@@ -64,8 +64,10 @@ if CONFIG['GNU_CXX']:
         '-Wno-empty-body',
         '-Wno-type-limits',
         '-Wno-unused-local-typedefs',
     ]
 
 # Force to build a static library, instead of a fake library, without
 # installing it in dist/lib.
 NO_EXPAND_LIBS = True
+
+ALLOW_COMPILER_WARNINGS = True

--- a/caps/moz.build
+++ b/caps/moz.build
@@ -46,10 +46,8 @@ UNIFIED_SOURCES += [
 LOCAL_INCLUDES += [
     '/dom/base',
     '/js/xpconnect/src',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
-
-FAIL_ON_WARNINGS = True

--- a/chrome/moz.build
+++ b/chrome/moz.build
@@ -35,10 +35,8 @@ GENERATED_INCLUDES += [
 LOCAL_INCLUDES += [
     '/netwerk/base',
     '/netwerk/protocol/res',
     '/xpcom/components'
 ]
 
 if CONFIG['MOZ_ENABLE_GTK']:
     CXXFLAGS += CONFIG['TK_CFLAGS']
-
-FAIL_ON_WARNINGS = True

--- a/config/config.mk
+++ b/config/config.mk
@@ -354,35 +354,37 @@ else
 HOST_CFLAGS	+= $(MOZ_OPTIMIZE_FLAGS)
 endif # MOZ_OPTIMIZE == 1
 endif # MOZ_OPTIMIZE
 endif # CROSS_COMPILE
 
 CFLAGS += $(MOZ_FRAMEPTR_FLAGS)
 CXXFLAGS += $(MOZ_FRAMEPTR_FLAGS)
 
-# Check for FAIL_ON_WARNINGS (Shorthand for Makefiles to request that we use
-# the 'warnings as errors' compile flags)
+# Check for ALLOW_COMPILER_WARNINGS (shorthand for Makefiles to request that we
+# *don't* use the warnings-as-errors compile flags)
 
-# NOTE: First, we clear FAIL_ON_WARNINGS[_DEBUG] if we're doing a Windows PGO
-# build, since WARNINGS_AS_ERRORS has been suspected of causing isuses in that
-# situation. (See bug 437002.)
+# Don't use warnings-as-errors in Windows PGO builds because it is suspected of
+# causing problems in that situation. (See bug 437002.)
 ifeq (WINNT_1,$(OS_ARCH)_$(MOZ_PROFILE_GENERATE)$(MOZ_PROFILE_USE))
-FAIL_ON_WARNINGS=
+ALLOW_COMPILER_WARNINGS=1
 endif # WINNT && (MOS_PROFILE_GENERATE ^ MOZ_PROFILE_USE)
 
-# Check for normal version of flag, and add WARNINGS_AS_ERRORS if it's set to 1.
-ifdef FAIL_ON_WARNINGS
-# Never treat warnings as errors in clang-cl, because it warns about many more
+# Don't use warnings-as-errors in clang-cl because it warns about many more
 # things than MSVC does.
-ifndef CLANG_CL
+ifdef CLANG_CL
+ALLOW_COMPILER_WARNINGS=1
+endif # CLANG_CL
+
+# Use warnings-as-errors if ALLOW_COMPILER_WARNINGS is not set to 1 (which
+# includes the case where it's undefined).
+ifneq (1,$(ALLOW_COMPILER_WARNINGS))
 CXXFLAGS += $(WARNINGS_AS_ERRORS)
 CFLAGS   += $(WARNINGS_AS_ERRORS)
-endif # CLANG_CL
-endif # FAIL_ON_WARNINGS
+endif # ALLOW_COMPILER_WARNINGS
 
 ifeq ($(OS_ARCH)_$(GNU_CC),WINNT_)
 #// Currently, unless USE_STATIC_LIBS is defined, the multithreaded
 #// DLL version of the RTL is used...
 #//
 #//------------------------------------------------------------------------
 ifdef USE_STATIC_LIBS
 RTL_FLAGS=-MT          # Statically linked multithreaded RTL

--- a/config/external/nss/moz.build
+++ b/config/external/nss/moz.build
@@ -40,8 +40,11 @@ else:
     Library('nss')
     USE_LIBS += [
         '/security/nss/lib/nss/nss3',
         '/security/nss/lib/smime/smime3',
         '/security/nss/lib/ssl/ssl3',
         '/security/nss/lib/util/nssutil3',
         'sqlite',
     ]
+
+# XXX: We should fix these warnings.
+ALLOW_COMPILER_WARNINGS = True

--- a/db/sqlite3/src/moz.build
+++ b/db/sqlite3/src/moz.build
@@ -4,16 +4,18 @@
 # 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/.
 NO_VISIBILITY_FLAGS = True
 
 EXPORTS += [
     'sqlite3.h',
 ]
 
+ALLOW_COMPILER_WARNINGS = True
+
 if CONFIG['MOZ_FOLD_LIBS']:
     # When folding libraries, sqlite is actually in the nss library.
     FINAL_LIBRARY = 'nss'
 else:
     # The final library is in config/external/sqlite
     FINAL_LIBRARY = 'sqlite'
 
 SOURCES += [

--- a/docshell/base/moz.build
+++ b/docshell/base/moz.build
@@ -60,18 +60,16 @@ UNIFIED_SOURCES += [
     'nsDocShellLoadInfo.cpp',
     'nsDocShellTransferableHooks.cpp',
     'nsDownloadHistory.cpp',
     'nsDSURIContentListener.cpp',
     'nsWebNavigationInfo.cpp',
     'SerializedLoadContext.cpp',
 ]
 
-FAIL_ON_WARNINGS = True
-
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
 LOCAL_INCLUDES += [
     '/docshell/shistory',
     '/dom/base',
     '/layout/base',
     '/layout/generic',

--- a/docshell/base/timeline/moz.build
+++ b/docshell/base/timeline/moz.build
@@ -15,15 +15,13 @@ EXPORTS.mozilla += [
 UNIFIED_SOURCES += [
     'AutoGlobalTimelineMarker.cpp',
     'AutoTimelineMarker.cpp',
     'ObservedDocShell.cpp',
     'TimelineConsumers.cpp',
     'TimelineMarker.cpp',
 ]
 
-FAIL_ON_WARNINGS = True
-
 FINAL_LIBRARY = 'xul'
 
 LOCAL_INCLUDES += [
     '/docshell/base'
 ]

--- a/docshell/build/moz.build
+++ b/docshell/build/moz.build
@@ -22,10 +22,8 @@ LOCAL_INCLUDES += [
 
 if CONFIG["MOZ_WIDGET_TOOLKIT"] == "cocoa":
     LOCAL_INCLUDES += ['/uriloader/exthandler/mac']
 
 FINAL_LIBRARY = 'xul'
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
-
-FAIL_ON_WARNINGS = True

--- a/docshell/shistory/moz.build
+++ b/docshell/shistory/moz.build
@@ -22,15 +22,13 @@ EXPORTS += [
 
 UNIFIED_SOURCES += [
     'nsSHEntry.cpp',
     'nsSHEntryShared.cpp',
     'nsSHistory.cpp',
     'nsSHTransaction.cpp',
 ]
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '/docshell/base',
 ]
 
 FINAL_LIBRARY = 'xul'

--- a/dom/activities/moz.build
+++ b/dom/activities/moz.build
@@ -26,17 +26,15 @@ EXTRA_COMPONENTS += [
     'ActivityWrapper.js',
 ]
 
 EXTRA_JS_MODULES += [
     'ActivitiesService.jsm',
     'ActivitiesServiceFilter.jsm',
 ]
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '/dom/base',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'

--- a/dom/alarm/moz.build
+++ b/dom/alarm/moz.build
@@ -23,17 +23,15 @@ EXTRA_COMPONENTS += [
     'AlarmsManager.manifest',
 ]
 
 EXTRA_JS_MODULES += [
     'AlarmDB.jsm',
     'AlarmService.jsm',
 ]
 
-FAIL_ON_WARNINGS = True
-
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
 
 MOCHITEST_MANIFESTS += ['test/mochitest.ini']
 
 XPCSHELL_TESTS_MANIFESTS += ['test/xpcshell.ini']

--- a/dom/animation/moz.build
+++ b/dom/animation/moz.build
@@ -25,11 +25,9 @@ UNIFIED_SOURCES += [
     'Animation.cpp',
     'AnimationEffectReadOnly.cpp',
     'AnimationTimeline.cpp',
     'DocumentTimeline.cpp',
     'KeyframeEffect.cpp',
     'PendingAnimationTracker.cpp',
 ]
 
-FAIL_ON_WARNINGS = True
-
 FINAL_LIBRARY = 'xul'

--- a/dom/apps/moz.build
+++ b/dom/apps/moz.build
@@ -49,15 +49,13 @@ EXTRA_PP_JS_MODULES += [
     'OperatorApps.jsm',
     'ScriptPreloader.jsm',
     'Webapps.jsm',
 ]
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
 
-FAIL_ON_WARNINGS = True
-
 FINAL_LIBRARY = 'xul'
 
 LOCAL_INCLUDES += [
     '/js/xpconnect/wrappers',
 ]

--- a/dom/archivereader/moz.build
+++ b/dom/archivereader/moz.build
@@ -19,17 +19,15 @@ UNIFIED_SOURCES += [
     'ArchiveRequest.cpp',
     'ArchiveZipEvent.cpp',
     'ArchiveZipFile.cpp',
 ]
 
 if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wshadow']
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '../base',
 ]
 
 FINAL_LIBRARY = 'xul'
 
 MOCHITEST_MANIFESTS += ['test/mochitest.ini']

--- a/dom/asmjscache/moz.build
+++ b/dom/asmjscache/moz.build
@@ -11,15 +11,13 @@ EXPORTS.mozilla.dom.asmjscache += [
 SOURCES += [
     'AsmJSCache.cpp'
 ]
 
 IPDL_SOURCES += [
     'PAsmJSCacheEntry.ipdl'
 ]
 
-FAIL_ON_WARNINGS = True
-
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
 
 MOCHITEST_MANIFESTS += ['test/mochitest.ini']

--- a/dom/audiochannel/moz.build
+++ b/dom/audiochannel/moz.build
@@ -16,13 +16,11 @@ EXPORTS += [
     'AudioChannelService.h',
 ]
 
 UNIFIED_SOURCES += [
     'AudioChannelAgent.cpp',
     'AudioChannelService.cpp',
 ]
 
-FAIL_ON_WARNINGS = True
-
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'

--- a/dom/base/moz.build
+++ b/dom/base/moz.build
@@ -404,18 +404,16 @@ if CONFIG['MOZ_BUILD_APP'] != 'mobile/an
         'SiteSpecificUserAgent.manifest',
     ]
 
 EXTRA_JS_MODULES += [
     'DOMRequestHelper.jsm',
     'IndexedDBHelper.jsm',
 ]
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '../battery',
     '../bluetooth/common',
     '../bluetooth/common/webapi',
     '../events',
     '../media',
     '../network',
     '../time',

--- a/dom/battery/moz.build
+++ b/dom/battery/moz.build
@@ -8,15 +8,13 @@ EXPORTS.mozilla.dom.battery += [
     'Constants.h',
     'Types.h',
 ]
 
 SOURCES += [
     'BatteryManager.cpp',
 ]
 
-FAIL_ON_WARNINGS = True
-
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
 
 MOCHITEST_MANIFESTS += ['test/mochitest.ini']

--- a/dom/bindings/moz.build
+++ b/dom/bindings/moz.build
@@ -34,18 +34,16 @@ EXPORTS.mozilla.dom += [
     'PrimitiveConversions.h',
     'RootedDictionary.h',
     'StructuredClone.h',
     'ToJSValue.h',
     'TypedArray.h',
     'UnionMember.h',
 ]
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '/dom/base',
     '/dom/battery',
     '/dom/bluetooth/common/webapi',
     '/dom/camera',
     '/dom/canvas',
     '/dom/geolocation',
     '/dom/html',

--- a/dom/bluetooth/moz.build
+++ b/dom/bluetooth/moz.build
@@ -147,17 +147,15 @@ EXPORTS.mozilla.dom.bluetooth += [
     'common/webapi/BluetoothPbapRequestHandle.h'
 ]
 IPDL_SOURCES += [
     'ipc/BluetoothTypes.ipdlh',
     'ipc/PBluetooth.ipdl',
     'ipc/PBluetoothRequest.ipdl'
 ]
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '../base',
     '../network',
     '../system/gonk'
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')

--- a/dom/broadcastchannel/moz.build
+++ b/dom/broadcastchannel/moz.build
@@ -24,9 +24,8 @@ LOCAL_INCLUDES += [
 ]
 
 MOCHITEST_MANIFESTS += ['tests/mochitest.ini']
 MOCHITEST_CHROME_MANIFESTS += ['tests/chrome.ini']
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'
-FAIL_ON_WARNINGS = True

--- a/dom/browser-element/moz.build
+++ b/dom/browser-element/moz.build
@@ -27,18 +27,16 @@ EXTRA_COMPONENTS += [
     'BrowserElementParent.js',
     'BrowserElementParent.manifest',
 ]
 
 EXTRA_JS_MODULES += [
     'BrowserElementPromptService.jsm',
 ]
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '../bluetooth',
     '/dom/html',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 FINAL_LIBRARY = 'xul'

--- a/dom/cache/moz.build
+++ b/dom/cache/moz.build
@@ -82,18 +82,16 @@ IPDL_SOURCES += [
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 LOCAL_INCLUDES += [
     '../workers',
 ]
 
-FAIL_ON_WARNINGS = True
-
 FINAL_LIBRARY = 'xul'
 
 MOCHITEST_MANIFESTS += [
     'test/mochitest/mochitest.ini',
 ]
 
 MOCHITEST_CHROME_MANIFESTS += [
     'test/mochitest/chrome.ini',

--- a/dom/camera/moz.build
+++ b/dom/camera/moz.build
@@ -56,18 +56,16 @@ if CONFIG['MOZ_B2G_CAMERA']:
         'CameraTestHardware.manifest',
     ]
 else:
     UNIFIED_SOURCES += [
         'FallbackCameraControl.cpp',
         'FallbackCameraManager.cpp',
     ]
 
-FAIL_ON_WARNINGS = True
-
 LOCAL_INCLUDES += [
     '../base',
     '/media/libyuv/include',
 ]
 
 include('/ipc/chromium/chromium-config.mozbuild')
 
 # Suppress some GCC warnings being treated as errors: