--- a/client.mk
+++ b/client.mk
@@ -377,9 +377,14 @@ cleansrcdir:
 	          -o -exec test ! -d {}/CVS \; \) -prune \
 	          -o \( -name '*.[ao]' -o -name '*.so' \) -type f -print`; \
 	   build/autoconf/clean-config.sh; \
 	fi;
 
 echo-variable-%:
 	@echo $($*)
 
+# This makefile doesn't support parallel execution. It does pass
+# MOZ_MAKE_FLAGS to sub-make processes, so they will correctly execute
+# in parallel.
+.NOTPARALLEL:
+
 .PHONY: checkout real_checkout depend build export libs alldep install clean realclean distclean cleansrcdir pull_all build_all clobber clobber_all pull_and_build_all everything configure preflight_all preflight postflight postflight_all