diff -up firefox-55.0/js/src/old-configure.in.rhbz-1219542-s390 firefox-55.0/js/src/old-configure.in

--- firefox-55.0/js/src/old-configure.in.rhbz-1219542-s390	2017-07-31 18:20:48.000000000 +0200

+++ firefox-55.0/js/src/old-configure.in	2017-08-02 14:31:32.190243669 +0200

@@ -541,7 +541,7 @@ case "$host" in

 *-linux*|*-kfreebsd*-gnu|*-gnu*)

     HOST_CFLAGS="$HOST_CFLAGS -DXP_UNIX"

-    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-O3}"

+    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-O1}"

     ;;

 *)

@@ -617,8 +617,8 @@ case "$target" in

 *-*linux*)

     if test "$GNU_CC" -o "$GNU_CXX"; then

-        MOZ_PGO_OPTIMIZE_FLAGS="-O3"

-        MOZ_OPTIMIZE_FLAGS="-O3"

+        MOZ_PGO_OPTIMIZE_FLAGS="-O1"

+        MOZ_OPTIMIZE_FLAGS="-O1"

         if test -z "$CLANG_CC"; then

            MOZ_OPTIMIZE_FLAGS="-freorder-blocks $MOZ_OPTIMIZE_FLAGS"

         fi