diff -r ac0c01689b40 -r deefc01c0e14 widget/xpwidgets/nsTransferable.cpp
--- a/widget/xpwidgets/nsTransferable.cpp	Thu Jan 15 15:59:08 2015 +0100
+++ b/widget/xpwidgets/nsTransferable.cpp	Thu Jan 15 21:03:48 2015 +0100
@@ -55,8 +55,6 @@
 }
 
 //-------------------------------------------------------------------------
-// The SetData method is overloaded to indicate private browsing mode
-// while achieving some semblance of (private) interface compatibility.
 //-------------------------------------------------------------------------
 void
 DataStruct::SetData ( nsISupports* aData, uint32_t aDataLen, bool aIsPrivBrowsing )
@@ -78,24 +76,6 @@
 
 //-------------------------------------------------------------------------
 void
-DataStruct::SetData ( nsISupports* aData, uint32_t aDataLen )
-{
-  // Now, check to see if we consider the data to be "too large"
-  if (aDataLen > kLargeDatasetSize) {
-    // if so, cache it to disk instead of memory
-    if ( NS_SUCCEEDED(WriteCache(aData, aDataLen)) )
-      return;
-    else
-			NS_WARNING("Oh no, couldn't write data to the cache file");   
-  } 
-
-  mData    = aData;
-  mDataLen = aDataLen;  
-}
-
-
-//-------------------------------------------------------------------------
-void
 DataStruct::GetData ( nsISupports** aData, uint32_t *aDataLen )
 {
   // check here to see if the data is cached on disk
@@ -439,14 +419,20 @@
     for (i = 0; i < mDataArray.Length(); ++i) {
       DataStruct& data = mDataArray.ElementAt(i);
       bool canConvert = false;
+      bool aIsPrivBrowsing = false;
       mFormatConv->CanConvert(aFlavor, data.GetFlavor().get(), &canConvert);
 
       if ( canConvert ) {
         nsCOMPtr<nsISupports> ConvertedData;
         uint32_t ConvertedLen;
         mFormatConv->Convert(aFlavor, aData, aDataLen, data.GetFlavor().get(), getter_AddRefs(ConvertedData), &ConvertedLen);
-        data.SetData(ConvertedData, ConvertedLen);
-        return NS_OK;
+        if (NS_SUCCEEDED(GetIsPrivateData(&aIsPrivBrowsing))) {
+          data.SetData(ConvertedData, ConvertedLen, aIsPrivBrowsing);
+          return NS_OK;
+        }
+        else {                     // call to GetIsPrivateData() failed,
+          return NS_ERROR_FAILURE; // we didn't SetData(), generic error
+        }
       }
     }
   }