diff --git a/netwerk/base/nsIncrementalStreamLoader.cpp b/netwerk/base/nsIncrementalStreamLoader.cpp
index a7298be3f..8890333f8 100644
--- a/netwerk/base/nsIncrementalStreamLoader.cpp
+++ b/netwerk/base/nsIncrementalStreamLoader.cpp
@@ -11,10 +11,7 @@
 
 #include <limits>
 
-nsIncrementalStreamLoader::nsIncrementalStreamLoader()
-  : mData(), mBytesConsumed(0)
-{
-}
+nsIncrementalStreamLoader::nsIncrementalStreamLoader() = default;
 
 nsIncrementalStreamLoader::~nsIncrementalStreamLoader()
 {
@@ -49,7 +46,7 @@ NS_IMPL_ISUPPORTS(nsIncrementalStreamLoader, nsIIncrementalStreamLoader,
 NS_IMETHODIMP
 nsIncrementalStreamLoader::GetNumBytesRead(uint32_t* aNumBytes)
 {
-  *aNumBytes = mBytesConsumed + mData.length();
+  *aNumBytes = mBytesRead;
   return NS_OK;
 }
 
@@ -180,7 +177,6 @@ nsIncrementalStreamLoader::WriteSegmentFun(nsIInputStream *inStr,
     }
   }
 
-  self->mBytesConsumed += consumedCount;
   *writeCount = count;
 
   return NS_OK;
@@ -198,6 +194,8 @@ nsIncrementalStreamLoader::OnDataAvailable(nsIRequest* request, nsISupports *ctx
   uint32_t countRead;
   nsresult rv = inStr->ReadSegments(WriteSegmentFun, this, count, &countRead);
   mRequest = nullptr;
+  NS_ENSURE_SUCCESS(rv, rv);
+  mBytesRead += countRead;
   return rv;
 }
 
diff --git a/netwerk/base/nsIncrementalStreamLoader.h b/netwerk/base/nsIncrementalStreamLoader.h
index f04d4a958..0e2b532e6 100644
--- a/netwerk/base/nsIncrementalStreamLoader.h
+++ b/netwerk/base/nsIncrementalStreamLoader.h
@@ -47,8 +47,9 @@ protected:
   // available.
   mozilla::Vector<uint8_t, 0> mData;
 
-  // Number of consumed bytes from the mData.
-  size_t  mBytesConsumed;
+  // Number of bytes read, which may differ from the number of bytes in mData,
+  // since we incrementally remove from there.
+  mozilla::Atomic<uint32_t, mozilla::Relaxed> mBytesRead;
 };
 
 #endif // nsIncrementalStreamLoader_h__
diff --git a/netwerk/base/nsStreamLoader.cpp b/netwerk/base/nsStreamLoader.cpp
index a73b038a7..9990d1882 100644
--- a/netwerk/base/nsStreamLoader.cpp
+++ b/netwerk/base/nsStreamLoader.cpp
@@ -54,7 +54,7 @@ NS_IMPL_ISUPPORTS(nsStreamLoader, nsIStreamLoader,
 NS_IMETHODIMP 
 nsStreamLoader::GetNumBytesRead(uint32_t* aNumBytes)
 {
-  *aNumBytes = mData.length();
+  *aNumBytes = mBytesRead;
   return NS_OK;
 }
 
@@ -150,7 +150,10 @@ nsStreamLoader::OnDataAvailable(nsIRequest* request, nsISupports *ctxt,
                                 uint64_t sourceOffset, uint32_t count)
 {
   uint32_t countRead;
-  return inStr->ReadSegments(WriteSegmentFun, this, count, &countRead);
+  nsresult rv = inStr->ReadSegments(WriteSegmentFun, this, count, &countRead);
+  NS_ENSURE_SUCCESS(rv, rv);
+  mBytesRead += countRead;
+  return NS_OK;
 }
 
 void
diff --git a/netwerk/base/nsStreamLoader.h b/netwerk/base/nsStreamLoader.h
index 671fc441f..b611a1aed 100644
--- a/netwerk/base/nsStreamLoader.h
+++ b/netwerk/base/nsStreamLoader.h
@@ -47,6 +47,8 @@ protected:
   nsCOMPtr<nsIRequest>              mRequest;
   nsCOMPtr<nsIRequestObserver>      mRequestObserver;
 
+  mozilla::Atomic<uint32_t, mozilla::Relaxed> mBytesRead;
+
   // Buffer to accumulate incoming data. We preallocate if contentSize is
   // available.
   mozilla::Vector<uint8_t, 0> mData;