diff --git a/dom/media/mediasource/ContainerParser.cpp b/dom/media/mediasource/ContainerParser.cpp
index afb3f8c0e2..98e7f89751 100644
--- a/dom/media/mediasource/ContainerParser.cpp
+++ b/dom/media/mediasource/ContainerParser.cpp
@@ -98,7 +98,13 @@ ContainerParser::InitData()
 MediaByteRange
 ContainerParser::MediaSegmentRange()
 {
-  return mCompleteByteRange;
+  return mCompleteMediaSegmentRange;
+}
+
+MediaByteRange
+ContainerParser::InitSegmentRange()
+{
+  return mCompleteInitSegmentRange;
 }
 
 class WebMContainerParser : public ContainerParser {
@@ -209,6 +215,7 @@ public:
           // Super unlikely OOM
           return false;
         }
+        mCompleteInitSegmentRange = MediaByteRange(0, mParser.mInitEndOffset);
         char* buffer = reinterpret_cast<char*>(mInitData->Elements());
         mResource->ReadFromCache(buffer, 0, mParser.mInitEndOffset);
         MSE_DEBUG(WebMContainerParser, "Stashed init of %u bytes.",
@@ -337,6 +344,7 @@ public:
     if (initSegment || !HasCompleteInitData()) {
       MediaByteRange& range = mParser->mInitRange;
       if (range.Length()) {
+        mCompleteInitSegmentRange = range;
         if (!mInitData->SetLength(range.Length())) {
           // Super unlikely OOM
           return false;
@@ -354,7 +362,7 @@ public:
     mp4_demuxer::Interval<mp4_demuxer::Microseconds> compositionRange =
       mParser->GetCompositionRange(byteRanges);
 
-    mCompleteByteRange = mParser->FirstCompleteMediaSegment();
+    mCompleteMediaSegmentRange = mParser->FirstCompleteMediaSegment();
     mResource->EvictData(mParser->mOffset, mParser->mOffset);
 
     if (compositionRange.IsNull()) {
diff --git a/dom/media/mediasource/ContainerParser.h b/dom/media/mediasource/ContainerParser.h
index 12c008828e..868f7dd0b8 100644
--- a/dom/media/mediasource/ContainerParser.h
+++ b/dom/media/mediasource/ContainerParser.h
@@ -55,6 +55,7 @@ public:
   // Return the byte range of the first complete media segment or an empty
   // range if not complete.
   MediaByteRange MediaSegmentRange();
+  MediaByteRange InitSegmentRange();
 
   static ContainerParser* CreateForMIMEType(const nsACString& aType);
 
@@ -62,7 +63,8 @@ protected:
   nsRefPtr<MediaLargeByteBuffer> mInitData;
   nsRefPtr<SourceBufferResource> mResource;
   bool mHasInitData;
-  MediaByteRange mCompleteByteRange;
+  MediaByteRange mCompleteMediaSegmentRange;
+  MediaByteRange mCompleteInitSegmentRange;
   const nsCString mType;
 };