diff --git a/dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp b/dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp
index 8e8262317..06547ae4c 100644
--- a/dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp
+++ b/dom/media/platforms/ffmpeg/FFmpegDataDecoder.cpp
@@ -8,6 +8,7 @@
 
 #include <string.h>
 #include <unistd.h>
+#include <sys/sysctl.h>
 
 #include "FFmpegLog.h"
 #include "FFmpegDataDecoder.h"
@@ -41,6 +42,7 @@ FFmpegDataDecoder<LIBAV_VER>::FFmpegDataDecoder(FlushableTaskQueue* aTaskQueue,
   , mMonitor("FFMpegaDataDecoder")
   , mIsFlushing(false)
   , mCodecParser(nullptr)
+  , mHQ(false)
 {
   MOZ_COUNT_CTOR(FFmpegDataDecoder);
 }
@@ -108,10 +110,27 @@ FFmpegDataDecoder<LIBAV_VER>::InitDecoder()
   // FFmpeg will call back to this to negotiate a video pixel format.
   mCodecContext->get_format = ChoosePixelFormat;
 
-  mCodecContext->thread_count = PR_GetNumberOfProcessors();
+  //mCodecContext->thread_count = PR_GetNumberOfProcessors();
+  // PR_GetNumberOfProcessors() lies on TenFourFox. Get from sysctl().
+  // TenFourFox issue 599.
+  int mib[2] = { CTL_HW, HW_NCPU };
+  size_t len = sizeof(mCodecContext->thread_count);
+  if (sysctl(mib, 2, &(mCodecContext->thread_count), &len, NULL, 0) == -1)
+    mCodecContext->thread_count = 1;
+#if DEBUG
+  fprintf(stderr, "FFmpeg using %d CPUs.\n", mCodecContext->thread_count);
+#endif
   mCodecContext->thread_type = FF_THREAD_SLICE | FF_THREAD_FRAME;
   mCodecContext->thread_safe_callbacks = false;
 
+  if (!mHQ) {
+    // Enable skipping the loop filter and FFmpeg's lightly documented
+    // non-spec-compliant mode. TenFourFox issue 599. This is pushed here
+    // because this is not on the main thread, so we assert.
+    mCodecContext->flags2 |= CODEC_FLAG2_FAST;
+    mCodecContext->skip_loop_filter = AVDISCARD_ALL;
+  }
+
   if (mExtraData) {
     mCodecContext->extradata_size = mExtraData->Length();
     // FFmpeg may use SIMD instructions to access the data which reads the
diff --git a/dom/media/platforms/ffmpeg/FFmpegDataDecoder.h b/dom/media/platforms/ffmpeg/FFmpegDataDecoder.h
index a64e716f4..cd0a04149 100644
--- a/dom/media/platforms/ffmpeg/FFmpegDataDecoder.h
+++ b/dom/media/platforms/ffmpeg/FFmpegDataDecoder.h
@@ -38,6 +38,7 @@ public:
 
   static AVCodec* FindAVCodec(AVCodecID aCodec);
 
+  bool mHQ; // TenFourFox high quality mode (issue 599)
 protected:
   // Flush and Drain operation, always run
   virtual void ProcessFlush();
diff --git a/dom/media/platforms/ffmpeg/FFmpegDecoderModule.h b/dom/media/platforms/ffmpeg/FFmpegDecoderModule.h
index f81f150e5..958a93712 100644
--- a/dom/media/platforms/ffmpeg/FFmpegDecoderModule.h
+++ b/dom/media/platforms/ffmpeg/FFmpegDecoderModule.h
@@ -19,14 +19,14 @@ class FFmpegDecoderModule : public PlatformDecoderModule
 {
 public:
   static already_AddRefed<PlatformDecoderModule>
-  Create()
+  Create(bool hq)
   {
-    RefPtr<PlatformDecoderModule> pdm = new FFmpegDecoderModule();
+    RefPtr<PlatformDecoderModule> pdm = new FFmpegDecoderModule(hq);
 
     return pdm.forget();
   }
 
-  FFmpegDecoderModule() {}
+  FFmpegDecoderModule(bool hq) : mHQ(hq) {}
   virtual ~FFmpegDecoderModule() {}
 
   already_AddRefed<MediaDataDecoder>
@@ -75,6 +75,7 @@ public:
     }
   }
 
+  bool mHQ; // TenFourFox high quality mode (issue 599).
 };
 
 } // namespace mozilla
diff --git a/dom/media/platforms/ffmpeg/FFmpegH264Decoder.cpp b/dom/media/platforms/ffmpeg/FFmpegH264Decoder.cpp
index c27ed3899..4b2df08ea 100644
--- a/dom/media/platforms/ffmpeg/FFmpegH264Decoder.cpp
+++ b/dom/media/platforms/ffmpeg/FFmpegH264Decoder.cpp
@@ -99,7 +99,7 @@ FFmpegH264Decoder<LIBAV_VER>::DecodeResult
 FFmpegH264Decoder<LIBAV_VER>::DoDecodeFrame(MediaRawData* aSample)
 {
   MOZ_ASSERT(mTaskQueue->IsCurrentThreadIn());
-  if (PR_Now() < sLockOutDueToOOM) {
+  if (MOZ_UNLIKELY(PR_Now() < sLockOutDueToOOM)) {
     // Halt further allocations.
     NS_WARNING("** FFMPEG LOCKED OUT DUE TO OUT OF MEMORY **");
     mCallback->Error();
@@ -152,7 +152,7 @@ FFmpegH264Decoder<LIBAV_VER>::DoDecodeFrame(MediaRawData* aSample,
                                             uint8_t* aData, int aSize)
 {
   MOZ_ASSERT(mTaskQueue->IsCurrentThreadIn());
-  if (PR_Now() < sLockOutDueToOOM) {
+  if (MOZ_UNLIKELY(PR_Now() < sLockOutDueToOOM)) {
     // Halt further allocations.
     NS_WARNING("** FFMPEG LOCKED OUT DUE TO OUT OF MEMORY **");
     mCallback->Error();
diff --git a/dom/media/platforms/ffmpeg/FFmpegRuntimeLinker.cpp b/dom/media/platforms/ffmpeg/FFmpegRuntimeLinker.cpp
index e84cb5bee..b503d7191 100644
--- a/dom/media/platforms/ffmpeg/FFmpegRuntimeLinker.cpp
+++ b/dom/media/platforms/ffmpeg/FFmpegRuntimeLinker.cpp
@@ -17,6 +17,9 @@
   #include <mach-o/dyld.h>
 #endif /* XP_DARWIN */
 
+/* TenFourFox issue 599 */
+static bool sHQ = false;
+
 namespace mozilla
 {
 
@@ -26,7 +29,7 @@ FFmpegRuntimeLinker::LinkStatus FFmpegRuntimeLinker::sLinkStatus =
 template <int V> class FFmpegDecoderModule
 {
 public:
-  static already_AddRefed<PlatformDecoderModule> Create();
+  static already_AddRefed<PlatformDecoderModule> Create(bool hq);
 };
 
 static const char* sLibs[] = {
@@ -175,6 +178,9 @@ FFmpegRuntimeLinker::Bind(const char* aLibName)
     return false;
   }
 
+  /* This is the only safe place to get preferences. TenFourFox issue 599. */
+  sHQ = Preferences::GetBool("tenfourfox.mp4.high_quality", false);
+
   int version;
   switch (major) {
     case 53:
@@ -243,11 +249,11 @@ FFmpegRuntimeLinker::CreateDecoderModule()
 
   RefPtr<PlatformDecoderModule> module;
   switch (major) {
-    case 53: module = FFmpegDecoderModule<53>::Create(); break;
-    case 54: module = FFmpegDecoderModule<54>::Create(); break;
+    case 53: module = FFmpegDecoderModule<53>::Create(sHQ); break;
+    case 54: module = FFmpegDecoderModule<54>::Create(sHQ); break;
     case 55:
-    case 56: module = FFmpegDecoderModule<55>::Create(); break;
-    case 57: module = FFmpegDecoderModule<57>::Create(); break;
+    case 56: module = FFmpegDecoderModule<55>::Create(sHQ); break;
+    case 57: module = FFmpegDecoderModule<57>::Create(sHQ); break;
     default: module = nullptr;
   }
   return module.forget();