diff --git a/media/ffvpx/README_MOZILLA b/media/ffvpx/README_MOZILLA
index 050c849d79ac..ae62c383958d 100644
--- a/media/ffvpx/README_MOZILLA
+++ b/media/ffvpx/README_MOZILLA
@@ -30,3 +30,10 @@ replace: s/HAVE_SYSCTL 1/HAVE_SYSCTL 0
 
 config_win32/64.h/asm:
 add to configure command: --toolchain=msvc
+
+23 Sept 2016: libavcodec/pthread_frame.c has been patched so as to avoid
+generating large numbers of warnings from TSan (Thread Sanitizer) when
+decoding vp9 streams.  This will likely be fixed upstream sometime soon.
+When resyncing with upstream, first un-apply the patch shown at
+https://bugzilla.mozilla.org/show_bug.cgi?id=1274256#c60, then resync,
+then assess whether the patch is still necessary.
diff --git a/media/ffvpx/libavcodec/pthread_frame.c b/media/ffvpx/libavcodec/pthread_frame.c
index 7ef5e9f6be92..a10fcbfe714d 100644
--- a/media/ffvpx/libavcodec/pthread_frame.c
+++ b/media/ffvpx/libavcodec/pthread_frame.c
@@ -43,9 +43,29 @@
 #include "libavutil/opt.h"
 #include "libavutil/thread.h"
 
+#if defined(MOZ_TSAN)
+typedef  _Atomic(int)  atomic_int;
+#else
+typedef  volatile int  atomic_int;
+#endif
+
 /**
  * Context used by codec threads and stored in their AVCodecInternal thread_ctx.
  */
+typedef enum {
+    STATE_INPUT_READY,          ///< Set when the thread is awaiting a packet.
+    STATE_SETTING_UP,           ///< Set before the codec has called ff_thread_finish_setup().
+    STATE_GET_BUFFER,           /**<
+                                 * Set when the codec calls get_buffer().
+                                 * State is returned to STATE_SETTING_UP afterwards.
+                                 */
+    STATE_GET_FORMAT,           /**<
+                                 * Set when the codec calls get_format().
+                                 * State is returned to STATE_SETTING_UP afterwards.
+                                 */
+    STATE_SETUP_FINISHED        ///< Set after the codec has called ff_thread_finish_setup().
+} State;
+
 typedef struct PerThreadContext {
     struct FrameThreadContext *parent;
 
@@ -66,19 +86,7 @@ typedef struct PerThreadContext {
     int     got_frame;              ///< The output of got_picture_ptr from the last avcodec_decode_video() call.
     int     result;                 ///< The result of the last codec decode/encode() call.
 
-    enum {
-        STATE_INPUT_READY,          ///< Set when the thread is awaiting a packet.
-        STATE_SETTING_UP,           ///< Set before the codec has called ff_thread_finish_setup().
-        STATE_GET_BUFFER,           /**<
-                                     * Set when the codec calls get_buffer().
-                                     * State is returned to STATE_SETTING_UP afterwards.
-                                     */
-        STATE_GET_FORMAT,           /**<
-                                     * Set when the codec calls get_format().
-                                     * State is returned to STATE_SETTING_UP afterwards.
-                                     */
-        STATE_SETUP_FINISHED        ///< Set after the codec has called ff_thread_finish_setup().
-    } state;
+    atomic_int state;
 
     /**
      * Array of frames passed to ff_thread_release_buffer().
@@ -358,7 +366,8 @@ static int submit_packet(PerThreadContext *p, AVPacket *avpkt)
             while (p->state == STATE_SETTING_UP)
                 pthread_cond_wait(&p->progress_cond, &p->progress_mutex);
 
-            switch (p->state) {
+            State p_state = (State)p->state;
+            switch (p_state) {
             case STATE_GET_BUFFER:
                 p->result = ff_get_buffer(p->avctx, p->requested_frame, p->requested_flags);
                 break;
@@ -471,7 +480,7 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
 void ff_thread_report_progress(ThreadFrame *f, int n, int field)
 {
     PerThreadContext *p;
-    volatile int *progress = f->progress ? (int*)f->progress->data : NULL;
+    atomic_int *progress = f->progress ? (atomic_int*)f->progress->data : NULL;
 
     if (!progress || progress[field] >= n) return;
 
@@ -489,7 +498,7 @@ void ff_thread_report_progress(ThreadFrame *f, int n, int field)
 void ff_thread_await_progress(ThreadFrame *f, int n, int field)
 {
     PerThreadContext *p;
-    volatile int *progress = f->progress ? (int*)f->progress->data : NULL;
+    atomic_int *progress = f->progress ? (atomic_int*)f->progress->data : NULL;
 
     if (!progress || progress[field] >= n) return;