--- a/dom/media/AbstractMediaDecoder.h
+++ b/dom/media/AbstractMediaDecoder.h
@@ -122,16 +122,20 @@ public:
   virtual void SetPlatformCanOffloadAudio(bool aCanOffloadAudio) {}
 
   // Called by Decoder/State machine to check audio offload condtions are met
   virtual bool CheckDecoderCanOffloadAudio() { return false; }
 
   // Called from HTMLMediaElement when owner document activity changes
   virtual void SetElementVisibility(bool aIsVisible) {}
 
+  // Called by some MediaDecoderReader to determine if we can rely
+  // on the resource length to limit reads.
+  virtual bool HasInitializationData() { return false; }
+
   // Stack based class to assist in notifying the frame statistics of
   // parsed and decoded frames. Use inside video demux & decode functions
   // to ensure all parsed and decoded frames are reported on all return paths.
   class AutoNotifyDecoded {
   public:
     AutoNotifyDecoded(AbstractMediaDecoder* aDecoder, uint32_t& aParsed, uint32_t& aDecoded)
       : mDecoder(aDecoder), mParsed(aParsed), mDecoded(aDecoded) {}
     ~AutoNotifyDecoded() {

--- a/dom/media/mediasource/SourceBufferDecoder.cpp
+++ b/dom/media/mediasource/SourceBufferDecoder.cpp
@@ -114,16 +114,22 @@ SourceBufferDecoder::SetMediaEndTime(int
 }
 
 void
 SourceBufferDecoder::UpdatePlaybackPosition(int64_t aTime)
 {
   MSE_DEBUG("SourceBufferDecoder(%p)::UpdatePlaybackPosition UNIMPLEMENTED", this);
 }
 
+bool
+SourceBufferDecoder::HasInitializationData()
+{
+  return true;
+}
+
 void
 SourceBufferDecoder::OnReadMetadataCompleted()
 {
   MSE_DEBUG("SourceBufferDecoder(%p)::OnReadMetadataCompleted UNIMPLEMENTED", this);
 }
 
 void
 SourceBufferDecoder::NotifyWaitingForResourcesStatusChanged()

--- a/dom/media/mediasource/SourceBufferDecoder.h
+++ b/dom/media/mediasource/SourceBufferDecoder.h
@@ -55,16 +55,17 @@ public:
   virtual void OnReadMetadataCompleted() MOZ_FINAL MOZ_OVERRIDE;
   virtual void QueueMetadata(int64_t aTime, MediaInfo* aInfo, MetadataTags* aTags) MOZ_FINAL MOZ_OVERRIDE;
   virtual void RemoveMediaTracks() MOZ_FINAL MOZ_OVERRIDE;
   virtual void SetMediaDuration(int64_t aDuration) MOZ_FINAL MOZ_OVERRIDE;
   virtual void SetMediaEndTime(int64_t aTime) MOZ_FINAL MOZ_OVERRIDE;
   virtual void SetMediaSeekable(bool aMediaSeekable) MOZ_FINAL MOZ_OVERRIDE;
   virtual void UpdateEstimatedMediaDuration(int64_t aDuration) MOZ_FINAL MOZ_OVERRIDE;
   virtual void UpdatePlaybackPosition(int64_t aTime) MOZ_FINAL MOZ_OVERRIDE;
+  virtual bool HasInitializationData() MOZ_FINAL MOZ_OVERRIDE;
 
   // SourceBufferResource specific interface below.
 
   // Warning: this mirrors GetBuffered in MediaDecoder, but this class's base is
   // AbstractMediaDecoder, which does not supply this interface.
   nsresult GetBuffered(dom::TimeRanges* aBuffered);
 
   void SetReader(MediaDecoderReader* aReader)

--- a/dom/media/webm/WebMReader.cpp
+++ b/dom/media/webm/WebMReader.cpp
@@ -282,17 +282,18 @@ nsresult WebMReader::ReadMetadata(MediaI
 {
   NS_ASSERTION(mDecoder->OnDecodeThread(), "Should be on decode thread.");
 
   nestegg_io io;
   io.read = webm_read;
   io.seek = webm_seek;
   io.tell = webm_tell;
   io.userdata = mDecoder;
-  int64_t maxOffset = -1;
+  int64_t maxOffset = mDecoder->HasInitializationData() ?
+    mDecoder->GetResource()->GetLength() : -1;
   int r = nestegg_init(&mContext, io, &webm_log, maxOffset);
   if (r == -1) {
     return NS_ERROR_FAILURE;
   }
 
   uint64_t duration = 0;
   r = nestegg_duration(mContext, &duration);
   if (r == 0) {