--- a/config/external/nss/Makefile.in
+++ b/config/external/nss/Makefile.in
@@ -249,16 +249,20 @@ DEFAULT_GMAKE_FLAGS += MODULE_INCLUDES='
 
 # Work around NSS's MAKE_OBJDIR being racy. See bug #836220
 DEFAULT_GMAKE_FLAGS += MAKE_OBJDIR='$$(INSTALL) -D $$(OBJDIR)'
 
 # Work around NSS adding IMPORT_LIBRARY to TARGETS with no rule for
 # it, creating race conditions. See bug #836220
 DEFAULT_GMAKE_FLAGS += TARGETS='$$(LIBRARY) $$(SHARED_LIBRARY) $$(PROGRAM)'
 
+ifeq ($(OS_TARGET)_$(MOZ_FOLD_LIBS),WINNT_1)
+MOZ_FOLD_LIBS_FLAGS += -D_NSPR_BUILD_=1
+endif
+
 ifdef MOZ_FOLD_LIBS_FLAGS
 DEFAULT_GMAKE_FLAGS += XCFLAGS='$(MOZ_FOLD_LIBS_FLAGS)'
 endif
 
 ifndef WARNINGS_AS_ERRORS
 DEFAULT_GMAKE_FLAGS += NSS_ENABLE_WERROR=0
 endif
 ifeq ($(OS_TARGET),Android)