diff -up firefox-64.0/build/unix/mozconfig.unix.pgo firefox-64.0/build/unix/mozconfig.unix
--- firefox-64.0/build/unix/mozconfig.unix.pgo	2019-01-03 15:23:16.792980384 +0100
+++ firefox-64.0/build/unix/mozconfig.unix	2019-01-03 15:24:29.978693550 +0100
@@ -6,6 +6,15 @@ if [ -n "$FORCE_GCC" ]; then
     CC="$TOOLTOOL_DIR/gcc/bin/gcc"
     CXX="$TOOLTOOL_DIR/gcc/bin/g++"
 
+    if [ -n "$MOZ_PGO" ]; then
+        if [ -z "$USE_ARTIFACT" ]; then
+            ac_add_options --enable-lto
+        fi
+        export AR="$topsrcdir/gcc/bin/gcc-ar"
+        export NM="$topsrcdir/gcc/bin/gcc-nm"
+        export RANLIB="$topsrcdir/gcc/bin/gcc-ranlib"
+    fi
+
     # We want to make sure we use binutils and other binaries in the tooltool
     # package.
     mk_add_options "export PATH=$TOOLTOOL_DIR/gcc/bin:$PATH"
diff -up firefox-64.0/extensions/spellcheck/src/moz.build.pgo firefox-64.0/extensions/spellcheck/src/moz.build
--- firefox-64.0/extensions/spellcheck/src/moz.build.pgo	2018-12-07 03:56:27.000000000 +0100
+++ firefox-64.0/extensions/spellcheck/src/moz.build	2019-01-03 15:21:32.793385074 +0100
@@ -28,3 +28,5 @@ EXPORTS.mozilla += [
 
 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     CXXFLAGS += ['-Wno-error=shadow']
+
+CXXFLAGS += ['-fno-devirtualize']
\ No newline at end of file