diff -r a63d609f5ebe -r ac0c01689b40 widget/xpwidgets/nsTransferable.cpp
--- a/widget/xpwidgets/nsTransferable.cpp	Thu Jan 15 15:55:04 2015 +0100
+++ b/widget/xpwidgets/nsTransferable.cpp	Thu Jan 15 15:59:08 2015 +0100
@@ -55,6 +55,28 @@
 }
 
 //-------------------------------------------------------------------------
+// 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 )
+{
+  // Now, check to see if we consider the data to be "too large"
+  // as well as ensuring that private browsing mode is disabled
+  if (aDataLen > kLargeDatasetSize && aIsPrivBrowsing == false) {
+    // 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::SetData ( nsISupports* aData, uint32_t aDataLen )
 {
@@ -398,11 +420,17 @@
 
   // first check our intrinsic flavors to see if one has been registered.
   uint32_t i = 0;
+  bool aIsPrivBrowsing = false;
   for (i = 0; i < mDataArray.Length(); ++i) {
     DataStruct& data = mDataArray.ElementAt(i);
     if ( data.GetFlavor().Equals(aFlavor) ) {
-      data.SetData ( aData, aDataLen );
-      return NS_OK;
+      if ( NS_SUCCEEDED(GetIsPrivateData(&aIsPrivBrowsing)) ) {
+        data.SetData ( aData, aDataLen, aIsPrivBrowsing );
+        return NS_OK;
+      }
+      else {                     // call to GetIsPrivateData() failed,
+        return NS_ERROR_FAILURE; // we didn't SetData(), generic error
+      }
     }
   }