diff --git a/www-client/firefox/firefox-64.0.ebuild b/www-client/firefox/firefox-64.0.ebuild
index 1a6c01d3a7a..f08b9dd4dcf 100644
--- a/www-client/firefox/firefox-64.0.ebuild
+++ b/www-client/firefox/firefox-64.0.ebuild
@@ -462,7 +462,7 @@ src_configure() {
 }
 
 src_compile() {
-	MOZ_MAKE_FLAGS="${MAKEOPTS}" SHELL="${SHELL:-${EPREFIX}/bin/bash}" MOZ_NOSPAM=1 \
+	MOZ_MAKE_FLAGS="${MAKEOPTS} -O" SHELL="${SHELL:-${EPREFIX}/bin/bash}" MOZ_NOSPAM=1 \
 	./mach build --verbose || die
 }