xpcshell is used only during firefox build, see bug 590658
--- firefox-48.0/config/rules.mk.orig	2016-08-18 02:01:43.000000000 +0300
+++ firefox-48.0/config/rules.mk	2016-08-18 09:45:50.262623009 +0300
@@ -688,6 +688,11 @@
 ifdef MOZ_POST_PROGRAM_COMMAND
 	$(MOZ_POST_PROGRAM_COMMAND) $@
 endif
+	@if test "$(PROGRAM)" = "xpcshell"; then \
+		if type -p "${prefix}/sbin/paxmark.sh" > /dev/null; then \
+			"${prefix}/sbin/paxmark.sh" m xpcshell; \
+		fi; \
+	fi
 
 $(HOST_PROGRAM): $(HOST_PROGOBJS) $(HOST_LIBS) $(HOST_EXTRA_DEPS) $(GLOBAL_DEPS)
 	$(REPORT_BUILD)