diff -r -U2 xulrunner-20.0/mozilla-release/config/Makefile.in xulrunner-20.0.new/mozilla-release/config/Makefile.in
--- xulrunner-20.0/mozilla-release/config/Makefile.in	2013-03-26 18:17:37.000000000 -0400
+++ xulrunner-20.0.new/mozilla-release/config/Makefile.in	2013-03-30 01:50:11.109526990 -0400
@@ -38,5 +38,5 @@
 
 ifdef GNU_CC
-MODULE_OPTIMIZE_FLAGS = -O3
+MODULE_OPTIMIZE_FLAGS = -Os
 endif
 
diff -r -U2 xulrunner-20.0/mozilla-release/configure.in xulrunner-20.0.new/mozilla-release/configure.in
--- xulrunner-20.0/mozilla-release/configure.in	2013-03-26 18:17:37.000000000 -0400
+++ xulrunner-20.0.new/mozilla-release/configure.in	2013-03-30 01:50:03.474547207 -0400
@@ -1848,5 +1848,5 @@
     HOST_CFLAGS="$HOST_CFLAGS -DXP_UNIX -DXP_MACOSX -DNO_X11"
     HOST_NSPR_MDCPUCFG='\"md/_darwin.cfg\"'
-    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-O3}"
+    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-Os}"
     ;;
 
@@ -1854,5 +1854,5 @@
     HOST_CFLAGS="$HOST_CFLAGS -DXP_UNIX"
     HOST_NSPR_MDCPUCFG='\"md/_linux.cfg\"'
-    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-O3}"
+    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-Os}"
     ;;
 
@@ -1961,5 +1961,5 @@
     MKSHLIB='$(CXX) $(CXXFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -o $@'
     MKCSHLIB='$(CC) $(CFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -o $@'
-    MOZ_OPTIMIZE_FLAGS="-O3"
+    MOZ_OPTIMIZE_FLAGS="-Os"
     _PEDANTIC=
     # Statically disable jemalloc on 10.5 and 32-bit 10.6.  See bug 702250.
@@ -2106,5 +2106,5 @@
             MOZ_OPTIMIZE_SIZE_TWEAK="-finline-limit=50"
         esac
-        MOZ_PGO_OPTIMIZE_FLAGS="-O3"
+        MOZ_PGO_OPTIMIZE_FLAGS="-Os"
         MOZ_OPTIMIZE_FLAGS="-Os -freorder-blocks $MOZ_OPTIMIZE_SIZE_TWEAK"
         MOZ_DEBUG_FLAGS="-g"
diff -r -U2 xulrunner-20.0/mozilla-release/js/src/config/Makefile.in xulrunner-20.0.new/mozilla-release/js/src/config/Makefile.in
--- xulrunner-20.0/mozilla-release/js/src/config/Makefile.in	2013-03-26 18:17:46.000000000 -0400
+++ xulrunner-20.0.new/mozilla-release/js/src/config/Makefile.in	2013-03-30 01:51:03.443862662 -0400
@@ -27,5 +27,5 @@
 
 ifdef GNU_CC
-MODULE_OPTIMIZE_FLAGS = -O3
+MODULE_OPTIMIZE_FLAGS = -Os
 endif
 
diff -r -U2 xulrunner-20.0/mozilla-release/js/src/configure.in xulrunner-20.0.new/mozilla-release/js/src/configure.in
--- xulrunner-20.0/mozilla-release/js/src/configure.in	2013-03-26 18:17:46.000000000 -0400
+++ xulrunner-20.0.new/mozilla-release/js/src/configure.in	2013-03-30 01:50:46.254052920 -0400
@@ -1443,5 +1443,5 @@
     HOST_CFLAGS="$HOST_CFLAGS -DXP_UNIX -DXP_MACOSX -DNO_X11"
     HOST_NSPR_MDCPUCFG='\"md/_darwin.cfg\"'
-    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-O3}"
+    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-Os}"
     ;;
 
@@ -1449,5 +1449,5 @@
     HOST_CFLAGS="$HOST_CFLAGS -DXP_UNIX"
     HOST_NSPR_MDCPUCFG='\"md/_linux.cfg\"'
-    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-O3}"
+    HOST_OPTIMIZE_FLAGS="${HOST_OPTIMIZE_FLAGS=-Os}"
     ;;
 
@@ -1554,5 +1554,5 @@
     MKSHLIB='$(CXX) $(CXXFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -o $@'
     MKCSHLIB='$(CC) $(CFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -o $@'
-    MOZ_OPTIMIZE_FLAGS="-O3 -fno-stack-protector"
+    MOZ_OPTIMIZE_FLAGS="-Os -fno-stack-protector"
     _PEDANTIC=
     CFLAGS="$CFLAGS -fno-common"
@@ -1648,5 +1648,5 @@
 
     MOZ_GFX_OPTIMIZE_MOBILE=1
-    MOZ_OPTIMIZE_FLAGS="-O3 -freorder-blocks -fno-reorder-functions"
+    MOZ_OPTIMIZE_FLAGS="-Os -freorder-blocks -fno-reorder-functions"
     # The Maemo builders don't know about this flag
     MOZ_ARM_VFP_FLAGS="-mfpu=vfp"
@@ -1668,6 +1668,6 @@
             MOZ_OPTIMIZE_SIZE_TWEAK="-finline-limit=50"
         esac
-        MOZ_PGO_OPTIMIZE_FLAGS="-O3"
-        MOZ_OPTIMIZE_FLAGS="-O3 -freorder-blocks $MOZ_OPTIMIZE_SIZE_TWEAK"
+        MOZ_PGO_OPTIMIZE_FLAGS="-Os"
+        MOZ_OPTIMIZE_FLAGS="-Os -freorder-blocks $MOZ_OPTIMIZE_SIZE_TWEAK"
         MOZ_DEBUG_FLAGS="-g"
     fi
diff -r -U2 xulrunner-20.0/mozilla-release/memory/jemalloc/src/configure xulrunner-20.0.new/mozilla-release/memory/jemalloc/src/configure
--- xulrunner-20.0/mozilla-release/memory/jemalloc/src/configure	2013-03-26 18:17:58.000000000 -0400
+++ xulrunner-20.0.new/mozilla-release/memory/jemalloc/src/configure	2013-03-30 01:51:38.533133311 -0400
@@ -5279,7 +5279,7 @@
 TCFLAGS="${CFLAGS}"
 if test "x${CFLAGS}" = "x" ; then
-  CFLAGS="-O3"
+  CFLAGS="-Os"
 else
-  CFLAGS="${CFLAGS} -O3"
+  CFLAGS="${CFLAGS} -Os"
 fi
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext
diff -r -U2 xulrunner-20.0/mozilla-release/memory/jemalloc/src/configure.ac xulrunner-20.0.new/mozilla-release/memory/jemalloc/src/configure.ac
--- xulrunner-20.0/mozilla-release/memory/jemalloc/src/configure.ac	2013-03-26 18:17:58.000000000 -0400
+++ xulrunner-20.0.new/mozilla-release/memory/jemalloc/src/configure.ac	2013-03-30 01:51:15.753872639 -0400
@@ -574,5 +574,5 @@
   if test "x${optimize}" = "xyes" ; then
     if test "x$GCC" = "xyes" ; then
-      JE_CFLAGS_APPEND([-O3])
+      JE_CFLAGS_APPEND([-Os])
       JE_CFLAGS_APPEND([-funroll-loops])
     elif test "x$je_cv_msvc" = "xyes" ; then