diff -up firefox-85.0/build/moz.configure/lto-pgo.configure.pgo firefox-85.0/build/moz.configure/lto-pgo.configure
--- firefox-85.0/build/moz.configure/lto-pgo.configure.pgo	2021-01-18 19:29:32.000000000 +0100
+++ firefox-85.0/build/moz.configure/lto-pgo.configure	2021-01-21 10:23:59.499111297 +0100
@@ -240,13 +240,13 @@ def lto(
                 cflags.append("-flto")
                 ldflags.append("-flto")
             else:
-                cflags.append("-flto=thin")
-                ldflags.append("-flto=thin")
+                cflags.append("-flto")
+                ldflags.append("-flto")
         elif c_compiler.type == "clang-cl":
             if len(value) and value[0].lower() == "full":
                 cflags.append("-flto")
             else:
-                cflags.append("-flto=thin")
+                cflags.append("-flto")
             # With clang-cl, -flto can only be used with -c or -fuse-ld=lld.
             # AC_TRY_LINKs during configure don't have -c, so pass -fuse-ld=lld.
             cflags.append("-fuse-ld=lld")
diff -up firefox-85.0/build/pgo/profileserver.py.pgo firefox-85.0/build/pgo/profileserver.py
--- firefox-85.0/build/pgo/profileserver.py.pgo	2021-01-18 19:29:32.000000000 +0100
+++ firefox-85.0/build/pgo/profileserver.py	2021-01-21 10:23:59.499111297 +0100
@@ -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
@@ -87,9 +87,22 @@ if __name__ == "__main__":
     locations = ServerLocations()
     locations.add_host(host="127.0.0.1", 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
@@ -212,6 +225,10 @@ 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-85.0/build/unix/mozconfig.unix.pgo firefox-85.0/build/unix/mozconfig.unix
--- firefox-85.0/build/unix/mozconfig.unix.pgo	2021-01-18 19:29:32.000000000 +0100
+++ firefox-85.0/build/unix/mozconfig.unix	2021-01-21 10:23:59.499111297 +0100
@@ -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-85.0/extensions/spellcheck/src/moz.build.pgo firefox-85.0/extensions/spellcheck/src/moz.build
--- firefox-85.0/extensions/spellcheck/src/moz.build.pgo	2021-01-18 19:29:38.000000000 +0100
+++ firefox-85.0/extensions/spellcheck/src/moz.build	2021-01-21 10:23:59.499111297 +0100
@@ -31,3 +31,5 @@ EXPORTS.mozilla += [
 
 if CONFIG["CC_TYPE"] in ("clang", "gcc"):
     CXXFLAGS += ["-Wno-error=shadow"]
+
+CXXFLAGS += ['-fno-devirtualize']
diff -up firefox-85.0/toolkit/components/terminator/nsTerminator.cpp.pgo firefox-85.0/toolkit/components/terminator/nsTerminator.cpp
--- firefox-85.0/toolkit/components/terminator/nsTerminator.cpp.pgo	2021-01-18 19:31:16.000000000 +0100
+++ firefox-85.0/toolkit/components/terminator/nsTerminator.cpp	2021-01-21 10:23:59.500111327 +0100
@@ -430,6 +430,11 @@ void nsTerminator::StartWatchdog() {
   }
 #endif
 
+  // Disable watchdog for PGO train builds - writting profile information at
+  // exit may take time and it is better to make build hang rather than
+  // silently produce poorly performing binary.
+  crashAfterMS = INT32_MAX;
+
   UniquePtr<Options> options(new Options());
   const PRIntervalTime ticksDuration = PR_MillisecondsToInterval(1000);
   options->crashAfterTicks = crashAfterMS / ticksDuration;