diff -up firefox-77.0.1/build/moz.configure/lto-pgo.configure.pgo firefox-77.0.1/build/moz.configure/lto-pgo.configure
--- firefox-77.0.1/build/moz.configure/lto-pgo.configure.pgo	2020-06-03 01:35:41.000000000 +0200
+++ firefox-77.0.1/build/moz.configure/lto-pgo.configure	2020-06-23 14:39:38.716560329 +0200
@@ -69,12 +69,14 @@ set_config('PGO_PROFILE_PATH', pgo_profi
 @imports(_from='__builtin__', _import='min')
 def pgo_flags(compiler, profdata, target_is_windows):
     if compiler.type == 'gcc':
+        #profile_use = '-fprofile-use=$(topobjdir)'
+        profile_use = '-fprofile-use'
         return namespace(
             gen_cflags=['-fprofile-generate'],
             gen_ldflags=['-fprofile-generate'],
-            use_cflags=['-fprofile-use', '-fprofile-correction',
+            use_cflags=[profile_use, '-fprofile-correction',
                         '-Wcoverage-mismatch'],
-            use_ldflags=['-fprofile-use'],
+            use_ldflags=[profile_use],
         )
 
     if compiler.type in ('clang-cl', 'clang'):
diff -up firefox-77.0.1/build/pgo/profileserver.py.pgo firefox-77.0.1/build/pgo/profileserver.py
--- firefox-77.0.1/build/pgo/profileserver.py.pgo	2020-06-03 03:04:50.000000000 +0200
+++ firefox-77.0.1/build/pgo/profileserver.py	2020-06-23 14:53:56.877736235 +0200
@@ -11,7 +11,7 @@ import glob
 import subprocess
 
 import mozcrash
-from mozbuild.base import MozbuildObject, BinaryNotFoundException
+from mozbuild.base import MozbuildObject, BinaryNotFoundException, BuildEnvironmentNotFoundException
 from mozfile import TemporaryDirectory
 from mozhttpd import MozHttpd
 from mozprofile import FirefoxProfile, Preferences
@@ -84,9 +84,22 @@ if __name__ == '__main__':
                        port=PORT,
                        options='primary,privileged')
 
-    old_profraw_files = glob.glob('*.profraw')
-    for f in old_profraw_files:
-        os.remove(f)
+    using_gcc = False
+    try:
+        if build.config_environment.substs.get('CC_TYPE') == 'gcc':
+            using_gcc = True
+    except BuildEnvironmentNotFoundException:
+        pass
+
+    if using_gcc:
+        for dirpath, _, filenames in os.walk('.'):
+            for f in filenames:
+                if f.endswith('.gcda'):
+                    os.remove(os.path.join(dirpath, f))
+    else:
+        old_profraw_files = glob.glob('*.profraw')
+        for f in old_profraw_files:
+            os.remove(f)
 
     with TemporaryDirectory() as profilePath:
         # TODO: refactor this into mozprofile
@@ -191,6 +204,11 @@ if __name__ == '__main__':
             print("Firefox exited successfully, but produced a crashreport")
             sys.exit(1)
 
+
+        print('Copying profile data....')
+        os.system('pwd');
+        os.system('tar cf profdata.tar.gz `find . -name "*.gcda"`; cd ..; tar xf instrumented/profdata.tar.gz;');
+
         llvm_profdata = env.get('LLVM_PROFDATA')
         if llvm_profdata:
             profraw_files = glob.glob('*.profraw')
diff -up firefox-77.0.1/build/unix/mozconfig.unix.pgo firefox-77.0.1/build/unix/mozconfig.unix
--- firefox-77.0.1/build/unix/mozconfig.unix.pgo	2020-06-03 01:35:41.000000000 +0200
+++ firefox-77.0.1/build/unix/mozconfig.unix	2020-06-23 14:39:38.716560329 +0200
@@ -6,6 +6,15 @@ if [ -n "$FORCE_GCC" ]; then
     CC="$MOZ_FETCHES_DIR/gcc/bin/gcc"
     CXX="$MOZ_FETCHES_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="$topsrcdir/gcc/bin/gcc-ranlib"
+    fi
+
     # We want to make sure we use binutils and other binaries in the tooltool
     # package.
     mk_add_options "export PATH=$MOZ_FETCHES_DIR/gcc/bin:$PATH"
diff -up firefox-77.0.1/config/config.mk.pgo firefox-77.0.1/config/config.mk
diff -up firefox-77.0.1/extensions/spellcheck/src/moz.build.pgo firefox-77.0.1/extensions/spellcheck/src/moz.build
--- firefox-77.0.1/extensions/spellcheck/src/moz.build.pgo	2020-06-03 01:35:48.000000000 +0200
+++ firefox-77.0.1/extensions/spellcheck/src/moz.build	2020-06-23 14:39:38.717560324 +0200
@@ -31,3 +31,5 @@ EXPORTS.mozilla += [
 
 if CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     CXXFLAGS += ['-Wno-error=shadow']
+
+CXXFLAGS += ['-fno-devirtualize']
\ No newline at end of file
diff -up firefox-77.0.1/python/mozbuild/mozbuild/build_commands.py.pgo firefox-77.0.1/python/mozbuild/mozbuild/build_commands.py
--- firefox-77.0.1/python/mozbuild/mozbuild/build_commands.py.pgo	2020-06-03 01:36:46.000000000 +0200
+++ firefox-77.0.1/python/mozbuild/mozbuild/build_commands.py	2020-06-23 14:39:38.717560324 +0200
@@ -114,7 +114,8 @@ class Build(MachCommandBase):
                 return status
 
             pgo_env = os.environ.copy()
-            pgo_env['LLVM_PROFDATA'] = instr.config_environment.substs.get('LLVM_PROFDATA')
+            if instr.config_environment.substs.get('CC_TYPE') != 'gcc':
+                pgo_env['LLVM_PROFDATA'] = instr.config_environment.substs.get('LLVM_PROFDATA')
             pgo_env['JARLOG_FILE'] = mozpath.join(orig_topobjdir, 'jarlog/en-US.log')
             pgo_cmd = [
                 instr.virtualenv_manager.python_path,