diff --git a/content/media/wmf/WMFDecoder.cpp b/content/media/wmf/WMFDecoder.cpp
index 9674b430cacc..8b4a101fadc8 100644
--- a/content/media/wmf/WMFDecoder.cpp
+++ b/content/media/wmf/WMFDecoder.cpp
@@ -25,6 +25,7 @@ MediaDecoderStateMachine* WMFDecoder::CreateStateMachine()
 bool
 WMFDecoder::IsMP3Supported()
 {
+  MOZ_ASSERT(NS_IsMainThread(), "Must be on main thread.");
   if (!MediaDecoder::IsWMFEnabled()) {
     return false;
   }
diff --git a/content/media/wmf/WMFReader.cpp b/content/media/wmf/WMFReader.cpp
index 8b112ef5685e..6991b46b9333 100644
--- a/content/media/wmf/WMFReader.cpp
+++ b/content/media/wmf/WMFReader.cpp
@@ -40,7 +40,8 @@ WMFReader::WMFReader(AbstractMediaDecoder* aDecoder)
     mVideoStride(0),
     mHasAudio(false),
     mHasVideo(false),
-    mCanSeek(false)
+    mCanSeek(false),
+    mIsMP3Enabled(WMFDecoder::IsMP3Supported())
 {
   NS_ASSERTION(NS_IsMainThread(), "Must be on main thread.");
   MOZ_COUNT_CTOR(WMFReader);
@@ -388,13 +389,13 @@ WMFReader::ConfigureVideoDecoder()
   return S_OK;
 }
 
-static void
-GetSupportedAudioCodecs(const GUID** aCodecs, uint32_t* aNumCodecs)
+void
+WMFReader::GetSupportedAudioCodecs(const GUID** aCodecs, uint32_t* aNumCodecs)
 {
   MOZ_ASSERT(aCodecs);
   MOZ_ASSERT(aNumCodecs);
 
-  if (WMFDecoder::IsMP3Supported()) {
+  if (mIsMP3Enabled) {
     static const GUID codecs[] = {
       MFAudioFormat_AAC,
       MFAudioFormat_MP3
diff --git a/content/media/wmf/WMFReader.h b/content/media/wmf/WMFReader.h
index 5c9919c90658..df994375913a 100644
--- a/content/media/wmf/WMFReader.h
+++ b/content/media/wmf/WMFReader.h
@@ -55,6 +55,7 @@ private:
   HRESULT ConfigureAudioDecoder();
   HRESULT ConfigureVideoDecoder();
   HRESULT ConfigureVideoFrameGeometry(IMFMediaType* aMediaType);
+  void GetSupportedAudioCodecs(const GUID** aCodecs, uint32_t* aNumCodecs);
 
   RefPtr<IMFSourceReader> mSourceReader;
   RefPtr<WMFByteStream> mByteStream;
@@ -74,6 +75,11 @@ private:
   bool mHasAudio;
   bool mHasVideo;
   bool mCanSeek;
+
+  // We can't call WMFDecoder::IsMP3Supported() on non-main threads, since it
+  // checks a pref, so we cache its value in mIsMP3Enabled and use that on
+  // the decode thread.
+  const bool mIsMP3Enabled;
 };
 
 } // namespace mozilla