diff --git a/js/src/Makefile.in b/js/src/Makefile.in
index 941b0f837712..881a7876fd98 100644
--- a/js/src/Makefile.in
+++ b/js/src/Makefile.in
@@ -727,6 +727,9 @@ ifdef JS_VERSION
 DEFINES		+= -DJS_VERSION=$(JS_VERSION)
 endif
 
+# We do not want to have obsolete NSPR functionality in threadsafe builds.
+DEFINES   += -DNO_NSPR_10_SUPPORT
+
 ifneq ($(findstring -L,$(NSPR_LIBS)),)
 NSPR_STATIC_PATH = $(subst -L,,$(findstring -L,$(NSPR_LIBS)))
 else