diff --git a/js/src/config/config.mk b/js/src/config/config.mk
index 0304dff7afc..40128a4209e 100644
--- a/js/src/config/config.mk
+++ b/js/src/config/config.mk
@@ -83,6 +83,11 @@ $(foreach x,$(CHECK_VARS),$(check-variable))
 
 core_abspath = $(if $(findstring :,$(1)),$(1),$(if $(filter /%,$(1)),$(1),$(CURDIR)/$(1)))
 
+nullstr :=
+space :=$(nullstr) # EOL
+
+core_winabspath = $(firstword $(subst /, ,$(call core_abspath,$(1)))):$(subst $(space),,$(patsubst %,\\%,$(wordlist 2,$(words $(subst /, ,$(call core_abspath,$(1)))), $(strip $(subst /, ,$(call core_abspath,$(1)))))))
+
 # FINAL_TARGET specifies the location into which we copy end-user-shipped
 # build products (typelibs, components, chrome).
 #