diff -up firefox-45.4.0esr/config/moz.build.forceO3 firefox-45.4.0esr/config/moz.build
--- firefox-45.4.0esr/config/moz.build.forceO3	2016-05-12 19:09:55.000000000 +0200
+++ firefox-45.4.0esr/config/moz.build	2016-09-21 14:33:37.225652630 +0200
@@ -33,7 +33,7 @@ PYTHON_UNITTEST_MANIFESTS += [
 ]
 
 if CONFIG['CC_TYPE'] in ('clang', 'gcc') and CONFIG['MOZ_OPTIMIZE']:
-    CFLAGS += ['-O3']
+    CFLAGS += ['-O2']
 
 HOST_DEFINES = {
     'UNICODE': True,
diff -up firefox-45.4.0esr/old-configure.in.forceO3 firefox-45.4.0esr/configure.in
--- firefox-45.4.0esr/old-configure.in.forceO3	2016-07-28 18:55:18.000000000 +0200
+++ firefox-45.4.0esr/old-configure.in	2016-09-21 14:32:38.203625716 +0200
@@ -820,7 +820,7 @@ 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=-O2}"
     ;;
 
 *)
@@ -1114,7 +1114,7 @@ case "$target" in
 esac
 
 if test -z "$MOZ_OPTIMIZE_FLAGS"; then
-    MOZ_OPTIMIZE_FLAGS="-O"
+    MOZ_OPTIMIZE_FLAGS=""
 fi
 
 AC_SUBST_LIST(MMX_FLAGS)
diff -up firefox-45.4.0esr/intl/icu/source/runConfigureICU.forceO3 firefox-45.4.0esr/intl/icu/source/runConfigureICU
--- firefox-45.4.0esr/intl/icu/source/runConfigureICU.forceO3	2016-05-12 19:09:55.000000000 +0200
+++ firefox-45.4.0esr/intl/icu/source/runConfigureICU	2016-09-21 14:30:54.670573553 +0200
@@ -237,16 +237,16 @@ case $platform in
         THE_COMP="the GNU C++"
         CC=gcc; export CC
         CXX=g++; export CXX
-        RELEASE_CFLAGS='-O3'
-        RELEASE_CXXFLAGS='-O3'
+        RELEASE_CFLAGS='-O2'
+        RELEASE_CXXFLAGS='-O2'
         DEBUG_CFLAGS='-g'
         DEBUG_CXXFLAGS='-g'
         ;;
     Linux*)
         THE_OS="Linux"
         THE_COMP="the clang or else GNU C++"
-        RELEASE_CFLAGS='-O3'
-        RELEASE_CXXFLAGS='-O3'
+        RELEASE_CFLAGS='-O2'
+        RELEASE_CXXFLAGS='-O2'
         DEBUG_CFLAGS='-g'
         DEBUG_CXXFLAGS='-g'
         ;;
@@ -380,7 +380,7 @@ then
     then
         case $CC in
             gcc|*/gcc|*-gcc-*|*/*-gcc-*)
-                RELEASE_CFLAGS=-O3
+                RELEASE_CFLAGS=-O2
                 ;;
         esac
     fi
@@ -392,7 +392,7 @@ then
     then
         case $CXX in
             g++|*/g++|*-g++-*|*/*-g++-*)
-                RELEASE_CXXFLAGS=-O3
+                RELEASE_CXXFLAGS=-O2
                 ;;
         esac
     fi
diff -up firefox-45.4.0esr/js/src/ctypes/libffi/m4/ax_cc_maxopt.m4.forceO3 firefox-45.4.0esr/js/src/ctypes/libffi/m4/ax_cc_maxopt.m4
--- firefox-45.4.0esr/js/src/ctypes/libffi/m4/ax_cc_maxopt.m4.forceO3	2016-05-12 19:09:56.000000000 +0200
+++ firefox-45.4.0esr/js/src/ctypes/libffi/m4/ax_cc_maxopt.m4	2016-09-21 14:35:34.100706257 +0200
@@ -138,7 +138,7 @@ if test "$ac_test_CFLAGS" != "set"; then
 
     gnu)
      # default optimization flags for gcc on all systems
-     CFLAGS="-O3 -fomit-frame-pointer"
+     CFLAGS="-O2 -fomit-frame-pointer"
 
      # -malign-double for x86 systems
      # LIBFFI -- DON'T DO THIS - CHANGES ABI
@@ -163,7 +163,7 @@ if test "$ac_test_CFLAGS" != "set"; then
 	echo "* (otherwise, a default of CFLAGS=-O3 will be used)    *"
 	echo "********************************************************"
 	echo ""
-        CFLAGS="-O3"
+        CFLAGS="-O2"
   fi
 
   AX_CHECK_COMPILE_FLAG($CFLAGS, [], [
diff -up firefox-45.4.0esr/security/nss/coreconf/mkdepend/Makefile.forceO3 firefox-45.4.0esr/security/nss/coreconf/mkdepend/Makefile
--- firefox-45.4.0esr/security/nss/coreconf/mkdepend/Makefile.forceO3	2016-05-12 19:09:51.000000000 +0200
+++ firefox-45.4.0esr/security/nss/coreconf/mkdepend/Makefile	2016-09-21 14:34:33.364678334 +0200
@@ -41,7 +41,7 @@ endif
 include $(DEPTH)/coreconf/rules.mk
 
 ifdef GNU_CC
-OPTIMIZER = -O3
+OPTIMIZER = -O2
 else
 ifeq ($(OS_ARCH),SunOS)
 OPTIMIZER = -fast