diff --git dom/media/flac/FlacDecoder.cpp dom/media/flac/FlacDecoder.cpp
index 53fc3c9937f7..b23771ab80fa 100644
--- mozilla/dom/media/flac/FlacDecoder.cpp
+++ mozilla/dom/media/flac/FlacDecoder.cpp
@@ -7,6 +7,7 @@
 #include "FlacDecoder.h"
 #include "MediaContainerType.h"
 #include "MediaPrefs.h"
+#include "PDMFactory.h"
 
 namespace mozilla {
 
@@ -15,6 +16,10 @@ FlacDecoder::IsEnabled()
 {
 #ifdef MOZ_FFVPX
   return MediaPrefs::FlacEnabled();
+#elif defined(MOZ_FFMPEG)
+  RefPtr<PDMFactory> platform = new PDMFactory();
+  return MediaPrefs::FlacEnabled() && platform->SupportsMimeType(NS_LITERAL_CSTRING("audio/flac"),
+                                    /* DecoderDoctorDiagnostics* */ nullptr);
 #else
   // Until bug 1295886 is fixed.
   return false;
diff --git media/ffvpx/README_MOZILLA media/ffvpx/README_MOZILLA
index 5d3a6037efe0..43144b726a92 100644
--- mozilla/media/ffvpx/README_MOZILLA
+++ mozilla/media/ffvpx/README_MOZILLA
@@ -17,14 +17,6 @@ configuration files were generated as follow using the configure script:
 config*:
 replace: /HAVE_(MALLOC_H|ARC4RANDOM|LOCALTIME_R|MEMALIGN|POSIX_MEMALIGN)/d
 
-config_darwin32.h:
-add to configure command: --disable-asm --disable-yasm --cc='clang -m32'
-
-config_unix32.h:
-add to configure command: --disable-asm --disable-yasm --cc='clang -m32'
-replace: s/HAVE_SYSCTL 1/HAVE_SYSCTL 0/ and s/HAVE_MEMALIGN 1/HAVE_MEMALIGN 0/ and s/HAVE_POSIX_MEMALIGN 1/HAVE_POSIX_MEMALIGN 0/
-
-
 config_unix64.h/config_unix64.asm:
 replace: s/HAVE_SYSCTL 1/HAVE_SYSCTL 0
 
diff --git media/ffvpx/config.h media/ffvpx/config.h
index dab01e05cd65..26ebe529bd98 100644
--- mozilla/media/ffvpx/config.h
+++ mozilla/media/ffvpx/config.h
@@ -27,17 +27,9 @@
 #define HAVE_LIBC_MSVCRT 0
 #endif
 #elif defined(XP_DARWIN)
-#if defined(HAVE_64BIT_BUILD)
 #include "config_darwin64.h"
-#else
-#include "config_darwin32.h"
-#endif
 #elif defined(XP_UNIX)
-#if defined(HAVE_64BIT_BUILD)
 #include "config_unix64.h"
-#else
-#include "config_unix32.h"
-#endif
 #endif
 #include "config_common.h"
 #endif // MOZ_FFVPX_CONFIG_H
diff --git old-configure.in old-configure.in
index b8ce71278548..410be683c2f6 100644
--- mozilla/old-configure.in
+++ mozilla/old-configure.in
@@ -3279,11 +3279,11 @@ dnl = FFmpeg's ffvpx configuration
 dnl ========================================================
 
 MOZ_FFVPX=
-case "$CPU_ARCH" in
-  x86)
+case "$OS_ARCH:$CPU_ARCH" in
+  WINNT:x86)
       MOZ_FFVPX=1
   ;;
-  x86_64)
+  *:x86_64)
       MOZ_FFVPX=1
   ;;
 esac