Index: thunderbird-52.0/mozilla/old-configure.in
===================================================================
--- thunderbird-52.0/mozilla/old-configure.in
+++ thunderbird-52.0/mozilla/old-configure.in	2017-04-12 12:41:15.074788631 -0400
@@ -953,7 +953,7 @@
 
 *-*linux*)
     if test "$GNU_CC" -o "$GNU_CXX"; then
-        MOZ_PGO_OPTIMIZE_FLAGS="-O3"
+        MOZ_PGO_OPTIMIZE_FLAGS="-O2"
         MOZ_OPTIMIZE_FLAGS=""
         if test -z "$CLANG_CC"; then
            MOZ_OPTIMIZE_FLAGS="-freorder-blocks $MOZ_OPTIMIZE_FLAGS"
Index: thunderbird-52.0/mozilla/js/src/old-configure.in
===================================================================
--- thunderbird-52.0/mozilla/js/src/old-configure.in
+++ thunderbird-52.0/mozilla/js/src/old-configure.in	2017-04-12 12:42:18.671889746 -0400
@@ -659,7 +659,7 @@
 
 *-*linux*)
     if test "$GNU_CC" -o "$GNU_CXX"; then
-        MOZ_PGO_OPTIMIZE_FLAGS="-O3"
+        MOZ_PGO_OPTIMIZE_FLAGS="-O2"
         MOZ_OPTIMIZE_FLAGS="-O2"
         if test -z "$CLANG_CC"; then
            MOZ_OPTIMIZE_FLAGS="-freorder-blocks $MOZ_OPTIMIZE_FLAGS"