diff -up firefox-123.0/build/moz.configure/lto-pgo.configure.pgo firefox-123.0/build/moz.configure/lto-pgo.configure
--- firefox-123.0/build/moz.configure/lto-pgo.configure.pgo	2024-02-14 08:18:40.000000000 +0100
+++ firefox-123.0/build/moz.configure/lto-pgo.configure	2024-02-19 10:51:40.900650193 +0100
@@ -130,7 +130,7 @@ def pgo_flags(compiler, linker, profdata
         return namespace(
             gen_cflags=["-fprofile-generate"],
             gen_ldflags=["-fprofile-generate"],
-            use_cflags=["-fprofile-use", "-fprofile-correction", "-Wcoverage-mismatch"],
+            use_cflags=["-fprofile-use", "-fprofile-correction", "-Wno-coverage-mismatch"],
             use_ldflags=["-fprofile-use"],
         )
 
diff -up firefox-123.0/build/pgo/profileserver.py.pgo firefox-123.0/build/pgo/profileserver.py
--- firefox-123.0/build/pgo/profileserver.py.pgo	2024-02-14 08:18:40.000000000 +0100
+++ firefox-123.0/build/pgo/profileserver.py	2024-02-19 10:51:40.900650193 +0100
@@ -11,7 +11,7 @@ import subprocess
 import sys
 
 import mozcrash
-from mozbuild.base import BinaryNotFoundException, MozbuildObject
+from mozbuild.base import BinaryNotFoundException, MozbuildObject, BuildEnvironmentNotFoundException
 from mozfile import TemporaryDirectory
 from mozhttpd import MozHttpd
 from mozprofile import FirefoxProfile, Preferences
@@ -98,9 +98,22 @@ 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
diff -up firefox-123.0/gfx/2d/moz.build.pgo firefox-123.0/gfx/2d/moz.build
--- firefox-123.0/gfx/2d/moz.build.pgo	2024-02-19 10:51:40.900650193 +0100
+++ firefox-123.0/gfx/2d/moz.build	2024-02-19 10:57:34.770832810 +0100
@@ -134,11 +134,11 @@ if CONFIG["INTEL_ARCHITECTURE"]:
     # The file uses SSE2 intrinsics, so it needs special compile flags on some
     # compilers.
     SOURCES["BlurSSE2.cpp"].flags += CONFIG["SSE2_FLAGS"]
-    SOURCES["ConvolutionFilterAVX2.cpp"].flags += ["-mavx2"]
+    SOURCES["ConvolutionFilterAVX2.cpp"].flags += ["-mavx2", "-fno-lto"]
     SOURCES["ConvolutionFilterSSE2.cpp"].flags += CONFIG["SSE2_FLAGS"]
     SOURCES["FilterProcessingSSE2.cpp"].flags += CONFIG["SSE2_FLAGS"]
     SOURCES["ImageScalingSSE2.cpp"].flags += CONFIG["SSE2_FLAGS"]
-    SOURCES["SwizzleAVX2.cpp"].flags += ["-mavx2"]
+    SOURCES["SwizzleAVX2.cpp"].flags += ["-mavx2", "-fno-lto"]
     SOURCES["SwizzleSSE2.cpp"].flags += CONFIG["SSE2_FLAGS"]
     SOURCES["SwizzleSSSE3.cpp"].flags += CONFIG["SSSE3_FLAGS"]
 elif CONFIG["TARGET_CPU"].startswith("mips"):
diff -up firefox-123.0/gfx/skia/generate_mozbuild.py.pgo firefox-123.0/gfx/skia/generate_mozbuild.py
--- firefox-123.0/gfx/skia/generate_mozbuild.py.pgo	2024-02-14 08:18:42.000000000 +0100
+++ firefox-123.0/gfx/skia/generate_mozbuild.py	2024-02-19 10:51:40.901650227 +0100
@@ -54,10 +54,10 @@ if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'wind
 if CONFIG['INTEL_ARCHITECTURE']:
     SOURCES['skia/src/opts/SkOpts_ssse3.cpp'].flags += ['-Dskvx=skvx_ssse3', '-mssse3']
     SOURCES['skia/src/opts/SkOpts_sse42.cpp'].flags += ['-Dskvx=skvx_sse42', '-msse4.2']
-    SOURCES['skia/src/opts/SkOpts_avx.cpp'].flags += ['-Dskvx=skvx_avx', '-mavx']
-    SOURCES['skia/src/opts/SkOpts_hsw.cpp'].flags += ['-Dskvx=skvx_hsw', '-mavx2', '-mf16c', '-mfma']
+    SOURCES['skia/src/opts/SkOpts_avx.cpp'].flags += ['-Dskvx=skvx_avx', '-mavx', '-fno-lto']
+    SOURCES['skia/src/opts/SkOpts_hsw.cpp'].flags += ['-Dskvx=skvx_hsw', '-mavx2', '-mf16c', '-mfma', '-fno-lto']
     if not CONFIG["MOZ_CODE_COVERAGE"]:
-        SOURCES['skia/src/opts/SkOpts_skx.cpp'].flags += ['-Dskvx=skvx_skx', '-mavx512f', '-mavx512dq', '-mavx512cd', '-mavx512bw', '-mavx512vl']
+        SOURCES['skia/src/opts/SkOpts_skx.cpp'].flags += ['-Dskvx=skvx_skx', '-mavx512f', '-mavx512dq', '-mavx512cd', '-mavx512bw', '-mavx512vl', '-fno-lto']
 elif CONFIG['TARGET_CPU'] == 'aarch64' and CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     SOURCES['skia/src/opts/SkOpts_crc32.cpp'].flags += ['-Dskvx=skvx_crc32', '-march=armv8-a+crc']
 
diff -up firefox-123.0/gfx/skia/moz.build.pgo firefox-123.0/gfx/skia/moz.build
--- firefox-123.0/gfx/skia/moz.build.pgo	2024-02-14 08:18:42.000000000 +0100
+++ firefox-123.0/gfx/skia/moz.build	2024-02-19 10:51:40.901650227 +0100
@@ -564,10 +564,10 @@ if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'wind
 if CONFIG['INTEL_ARCHITECTURE']:
     SOURCES['skia/src/opts/SkOpts_ssse3.cpp'].flags += ['-Dskvx=skvx_ssse3', '-mssse3']
     SOURCES['skia/src/opts/SkOpts_sse42.cpp'].flags += ['-Dskvx=skvx_sse42', '-msse4.2']
-    SOURCES['skia/src/opts/SkOpts_avx.cpp'].flags += ['-Dskvx=skvx_avx', '-mavx']
-    SOURCES['skia/src/opts/SkOpts_hsw.cpp'].flags += ['-Dskvx=skvx_hsw', '-mavx2', '-mf16c', '-mfma']
+    SOURCES['skia/src/opts/SkOpts_avx.cpp'].flags += ['-Dskvx=skvx_avx', '-mavx', '-fno-lto']
+    SOURCES['skia/src/opts/SkOpts_hsw.cpp'].flags += ['-Dskvx=skvx_hsw', '-mavx2', '-mf16c', '-mfma', '-fno-lto']
     if not CONFIG["MOZ_CODE_COVERAGE"]:
-        SOURCES['skia/src/opts/SkOpts_skx.cpp'].flags += ['-Dskvx=skvx_skx', '-mavx512f', '-mavx512dq', '-mavx512cd', '-mavx512bw', '-mavx512vl']
+        SOURCES['skia/src/opts/SkOpts_skx.cpp'].flags += ['-Dskvx=skvx_skx', '-mavx512f', '-mavx512dq', '-mavx512cd', '-mavx512bw', '-mavx512vl', '-fno-lto']
 elif CONFIG['TARGET_CPU'] == 'aarch64' and CONFIG['CC_TYPE'] in ('clang', 'gcc'):
     SOURCES['skia/src/opts/SkOpts_crc32.cpp'].flags += ['-Dskvx=skvx_crc32', '-march=armv8-a+crc']
 
diff -up firefox-123.0/toolkit/components/terminator/nsTerminator.cpp.pgo firefox-123.0/toolkit/components/terminator/nsTerminator.cpp
--- firefox-123.0/toolkit/components/terminator/nsTerminator.cpp.pgo	2024-02-14 08:18:56.000000000 +0100
+++ firefox-123.0/toolkit/components/terminator/nsTerminator.cpp	2024-02-19 10:51:40.901650227 +0100
@@ -460,6 +460,11 @@ 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());
   // crashAfterTicks is guaranteed to be > 0 as
   // crashAfterMS >= ADDITIONAL_WAIT_BEFORE_CRASH_MS >> HEARTBEAT_INTERVAL_MS