--- a/dom/media/webm/WebMBufferedParser.cpp
+++ b/dom/media/webm/WebMBufferedParser.cpp
@@ -28,17 +28,17 @@ VIntLength(unsigned char aFirstByte, uin
   }
   if (aMask) {
     *aMask = mask;
   }
   NS_ASSERTION(count >= 1 && count <= 8, "Insane VInt length.");
   return count;
 }
 
-void WebMBufferedParser::Append(const unsigned char* aBuffer, uint32_t aLength,
+bool WebMBufferedParser::Append(const unsigned char* aBuffer, uint32_t aLength,
                                 nsTArray<WebMTimeDataOffset>& aMapping,
                                 ReentrantMonitor& aReentrantMonitor)
 {
   static const uint32_t EBML_ID = 0x1a45dfa3;
   static const uint32_t SEGMENT_ID = 0x18538067;
   static const uint32_t SEGINFO_ID = 0x1549a966;
   static const uint32_t TRACKS_ID = 0x1654AE6B;
   static const uint32_t CLUSTER_ID = 0x1f43b675;
@@ -158,17 +158,19 @@ void WebMBufferedParser::Append(const un
         mVInt.mValue <<= 8;
         mVInt.mValue |= *p++;
         mVIntLeft -= 1;
       } else {
         mState = mNextState;
       }
       break;
     case READ_TIMECODESCALE:
-      MOZ_ASSERT(mGotTimecodeScale);
+      if (!mGotTimecodeScale) {
+        return false;
+      }
       mTimecodeScale = mVInt.mValue;
       mState = READ_ELEMENT_ID;
       break;
     case READ_CLUSTER_TIMECODE:
       mClusterTimecode = mVInt.mValue;
       mState = READ_ELEMENT_ID;
       break;
     case READ_BLOCK_TIMECODE:
@@ -182,17 +184,19 @@ void WebMBufferedParser::Append(const un
         {
           ReentrantMonitorAutoEnter mon(aReentrantMonitor);
           int64_t endOffset = mBlockOffset + mBlockSize +
                               mElement.mID.mLength + mElement.mSize.mLength;
           uint32_t idx = aMapping.IndexOfFirstElementGt(endOffset);
           if (idx == 0 || aMapping[idx - 1] != endOffset) {
             // Don't insert invalid negative timecodes.
             if (mBlockTimecode >= 0 || mClusterTimecode >= uint16_t(abs(mBlockTimecode))) {
-              MOZ_ASSERT(mGotTimecodeScale);
+              if (!mGotTimecodeScale) {
+                return false;
+              }
               uint64_t absTimecode = mClusterTimecode + mBlockTimecode;
               absTimecode *= mTimecodeScale;
               // Avoid creating an entry if the timecode is out of order
               // (invalid according to the WebM specification) so that
               // ordering invariants of aMapping are not violated.
               if (idx == 0 ||
                   aMapping[idx - 1].mTimecode <= absTimecode ||
                   (idx + 1 < aMapping.Length() &&
@@ -243,16 +247,18 @@ void WebMBufferedParser::Append(const un
         mState = READ_ELEMENT_ID;
       }
       break;
     }
   }
 
   NS_ASSERTION(p == aBuffer + aLength, "Must have parsed to end of data.");
   mCurrentOffset += aLength;
+
+  return true;
 }
 
 int64_t
 WebMBufferedParser::EndSegmentOffset(int64_t aOffset)
 {
   if (mLastInitStartOffset > aOffset || mClusterOffset > aOffset) {
     return std::min(mLastInitStartOffset >= 0 ? mLastInitStartOffset : INT64_MAX,
                     mClusterOffset >= 0 ? mClusterOffset : INT64_MAX);

--- a/dom/media/webm/WebMBufferedParser.h
+++ b/dom/media/webm/WebMBufferedParser.h
@@ -91,17 +91,18 @@ struct WebMBufferedParser
   void SetTimecodeScale(uint32_t aTimecodeScale) {
     mTimecodeScale = aTimecodeScale;
     mGotTimecodeScale = true;
   }
 
   // Steps the parser through aLength bytes of data.  Always consumes
   // aLength bytes.  Updates mCurrentOffset before returning.  Acquires
   // aReentrantMonitor before using aMapping.
-  void Append(const unsigned char* aBuffer, uint32_t aLength,
+  // Returns false if an error was encountered.
+  bool Append(const unsigned char* aBuffer, uint32_t aLength,
               nsTArray<WebMTimeDataOffset>& aMapping,
               ReentrantMonitor& aReentrantMonitor);
 
   bool operator==(int64_t aOffset) const {
     return mCurrentOffset == aOffset;
   }
 
   bool operator<(int64_t aOffset) const {