diff --git a/content/base/src/nsXMLHttpRequest.cpp b/content/base/src/nsXMLHttpRequest.cpp
index 2904ebe88355..f15b00d98350 100644
--- a/content/base/src/nsXMLHttpRequest.cpp
+++ b/content/base/src/nsXMLHttpRequest.cpp
@@ -1690,7 +1690,7 @@ nsXMLHttpRequest::StreamReaderFunc(nsIInputStream* in,
   return rv;
 }
 
-void nsXMLHttpRequest::CreateResponseBlob(nsIRequest *request)
+bool nsXMLHttpRequest::CreateResponseBlob(nsIRequest *request)
 {
   nsCOMPtr<nsIFile> file;
   nsCOMPtr<nsICachingChannel> cc(do_QueryInterface(request));
@@ -1702,18 +1702,27 @@ void nsXMLHttpRequest::CreateResponseBlob(nsIRequest *request)
       fc->GetFile(getter_AddRefs(file));
     }
   }
+  bool fromFile = false;
   if (file) {
     nsCAutoString contentType;
     mChannel->GetContentType(contentType);
     nsCOMPtr<nsISupports> cacheToken;
     if (cc) {
       cc->GetCacheToken(getter_AddRefs(cacheToken));
+      // We need to call IsFromCache to determine whether the response is
+      // fully cached (i.e. whether we can skip reading the response).
+      cc->IsFromCache(&fromFile);
+    } else {
+      // If the response is coming from the local resource, we can skip
+      // reading the response unconditionally.
+      fromFile = true;
     }
 
     mResponseBlob =
       new nsDOMFileFile(file, NS_ConvertASCIItoUTF16(contentType), cacheToken);
     mResponseBody.Truncate();
   }
+  return fromFile;
 }
 
 NS_IMETHODIMP
@@ -1727,12 +1736,17 @@ nsXMLHttpRequest::OnDataAvailable(nsIRequest *request,
 
   NS_ABORT_IF_FALSE(mContext.get() == ctxt,"start context different from OnDataAvailable context");
 
-  if (mResponseType == XML_HTTP_RESPONSE_TYPE_BLOB && !mResponseBlob) {
-    CreateResponseBlob(request);
-  }
-
   mProgressSinceLastProgressEvent = true;
 
+  if (mResponseType == XML_HTTP_RESPONSE_TYPE_BLOB && !mResponseBlob) {
+    if (CreateResponseBlob(request)) {
+      // We don't have to read from the local file for the blob response
+      mResponseBlob->GetSize(&mLoadTransferred);
+      ChangeState(XML_HTTP_REQUEST_LOADING);
+      return request->Cancel(NS_OK);
+    }
+  }
+
   PRUint32 totalRead;
   nsresult rv = inStr->ReadSegments(nsXMLHttpRequest::StreamReaderFunc,
                                     (void*)this, count, &totalRead);
diff --git a/content/base/src/nsXMLHttpRequest.h b/content/base/src/nsXMLHttpRequest.h
index 57d756a7c5f7..9412b03deeca 100644
--- a/content/base/src/nsXMLHttpRequest.h
+++ b/content/base/src/nsXMLHttpRequest.h
@@ -217,7 +217,7 @@ protected:
                 PRUint32 *writeCount);
   nsresult CreateResponseParsedJSON(JSContext* aCx);
   nsresult CreateResponseArrayBuffer(JSContext* aCx);
-  void CreateResponseBlob(nsIRequest *request);
+  bool CreateResponseBlob(nsIRequest *request);
   // Change the state of the object with this. The broadcast argument
   // determines if the onreadystatechange listener should be called.
   nsresult ChangeState(PRUint32 aState, bool aBroadcast = true);