--- a/configure.in
+++ b/configure.in
@@ -2136,29 +2136,32 @@ ia64*-hpux*)
             CFLAGS="$CFLAGS -FS"
             CXXFLAGS="$CXXFLAGS -FS"
         fi
         # khuey says we can safely ignore MSVC warning C4251
         # MSVC warning C4244 (implicit type conversion may lose data) warns
         # and requires workarounds for perfectly valid code.  Also, GCC/clang
         # don't warn about it by default. So for consistency/sanity, we turn
         # it off on MSVC, too.
+        # MSVC warning C4267 warns for narrowing type conversions from size_t
+        # to 32-bit integer types on 64-bit platforms.  Since this is virtually
+        # the same thing as C4244, we disable C4267, too.
         # MSVC warning C4345 warns of newly conformant behavior as of VS2003.
         # MSVC warning C4351 warns of newly conformant behavior as of VS2005.
         # MSVC warning C4482 warns when an enum value is refered specifing the
         # name of the enum itself.  This behavior is allowed in C++11, and the
         # warning has been removed in VS2012.
         # MSVC warning C4800 warns when a value is implicitly cast to bool,
         # because this also forces narrowing to a single byte, which can be a
         # perf hit.  But this matters so little in practice (and often we want
         # that behavior) that it's better to turn it off.
         # MSVC warning C4819 warns some UTF-8 characters (e.g. copyright sign)
         # on non-Western system locales even if it is in a comment.
-        CFLAGS="$CFLAGS -wd4244 -wd4819"
-        CXXFLAGS="$CXXFLAGS -wd4251 -wd4244 -wd4345 -wd4351 -wd4482 -wd4800 -wd4819"
+        CFLAGS="$CFLAGS -wd4244 -wd4267 -wd4819"
+        CXXFLAGS="$CXXFLAGS -wd4251 -wd4244 -wd4267 -wd4345 -wd4351 -wd4482 -wd4800 -wd4819"
         # make 'foo == bar;' error out
         CFLAGS="$CFLAGS -we4553"
         CXXFLAGS="$CXXFLAGS -we4553"
         LIBS="$LIBS kernel32.lib user32.lib gdi32.lib winmm.lib wsock32.lib advapi32.lib secur32.lib netapi32.lib"
         MOZ_DEBUG_LDFLAGS='-DEBUG -DEBUGTYPE:CV'
         WARNINGS_AS_ERRORS='-WX'
         MOZ_OPTIMIZE_FLAGS='-O1 -Oi'
         MOZ_FIX_LINK_PATHS=

--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -1696,18 +1696,21 @@ ia64*-hpux*)
             CFLAGS="$CFLAGS -FS"
             CXXFLAGS="$CXXFLAGS -FS"
         fi
         # khuey says we can safely ignore MSVC warning C4251
         # MSVC warning C4244 (implicit type conversion may lose data) warns
         # and requires workarounds for perfectly valid code.  Also, GCC/clang
         # don't warn about it by default. So for consistency/sanity, we turn
         # it off on MSVC, too.
-        CFLAGS="$CFLAGS -wd4244"
-        CXXFLAGS="$CXXFLAGS -wd4244 -wd4251"
+        # MSVC warning C4267 warns for narrowing type conversions from size_t
+        # to 32-bit integer types on 64-bit platforms.  Since this is virtually
+        # the same thing as C4244, we disable C4267, too.
+        CFLAGS="$CFLAGS -wd4244 -wd4267"
+        CXXFLAGS="$CXXFLAGS -wd4244 -wd4267 -wd4251"
         # make 'foo == bar;' error out
         CFLAGS="$CFLAGS -we4553"
         CXXFLAGS="$CXXFLAGS -we4553"
         LIBS="$LIBS kernel32.lib user32.lib gdi32.lib winmm.lib wsock32.lib advapi32.lib psapi.lib"
         MOZ_DEBUG_LDFLAGS='-DEBUG -DEBUGTYPE:CV'
         WARNINGS_AS_ERRORS='-WX'
         MOZ_OPTIMIZE_FLAGS="-O2"
         MOZ_FIX_LINK_PATHS=