diff -r 141e0f1194b1 -r 97036ab72558 widget/xpwidgets/nsTransferable.cpp
--- a/widget/xpwidgets/nsTransferable.cpp	Wed Dec 31 07:16:47 2014 +0100
+++ b/widget/xpwidgets/nsTransferable.cpp	Tue Jan 06 21:39:09 2015 +0100
@@ -14,6 +14,7 @@
 
  
 #include "nsTransferable.h"
+#include "mozilla/Preferences.h"
 #include "nsString.h"
 #include "nsReadableUtils.h"
 #include "nsTArray.h"
@@ -58,8 +59,11 @@
 void
 DataStruct::SetData ( nsISupports* aData, uint32_t aDataLen )
 {
+  // First, prepare for conditional caching according to isolation mode
+  int32_t isolationState = mozilla::Preferences::GetInt("privacy.thirdparty.isolate");
+
   // Now, check to see if we consider the data to be "too large"
-  if (aDataLen > kLargeDatasetSize) {
+  if (aDataLen > kLargeDatasetSize && isolationState == 0) {
     // if so, cache it to disk instead of memory
     if ( NS_SUCCEEDED(WriteCache(aData, aDataLen)) )
       return;