diff --git a/mozilla/string/public/nsSharedBufferList.h b/mozilla/string/public/nsSharedBufferList.h
index 335d982cd4d..5c160296280 100755
--- a/mozilla/string/public/nsSharedBufferList.h
+++ b/mozilla/string/public/nsSharedBufferList.h
@@ -91,6 +91,12 @@ class NS_COM nsSharedBufferList
                 mutable_this->set_refcount( get_refcount()-1 );
               }
 
+            PRBool
+            IsSingleAllocationWithBuffer() const
+              {
+                return (mFlags & kIsSingleAllocationWithBuffer);
+              }
+
             Buffer* mPrev;
             Buffer* mNext;
 
diff --git a/mozilla/string/public/nsSlidingString.h b/mozilla/string/public/nsSlidingString.h
index 5c4901319b0..0f7a34248a6 100755
--- a/mozilla/string/public/nsSlidingString.h
+++ b/mozilla/string/public/nsSlidingString.h
@@ -33,6 +33,11 @@
 #include "nsSharedBufferList.h"
 #endif
 
+/*
+ * nsFreeProc : A pointer to the free function used by nsSlidingString
+ */
+typedef void nsFreeProc(void *ptr, void *clientdata);
+
 
   /**
    * Maintains the sequence from the prev-most referenced buffer to the last buffer.
@@ -54,7 +59,10 @@ class NS_COM nsSlidingSharedBufferList
     : public nsSharedBufferList
   {
     public:
-      nsSlidingSharedBufferList( Buffer* aBuffer ) : nsSharedBufferList(aBuffer), mRefCount(0) { }
+      nsSlidingSharedBufferList( Buffer* aBuffer )
+          : nsSharedBufferList(aBuffer), mRefCount(0), mFreeProc(nsnull), mClientData(nsnull) { }
+      nsSlidingSharedBufferList( Buffer* aBuffer, nsFreeProc *aFreeproc, void *aClientData = NULL )
+          : nsSharedBufferList(aBuffer), mRefCount(0), mFreeProc(aFreeproc), mClientData(aClientData) { }
 
       void  AcquireReference()    { ++mRefCount; }
       void  ReleaseReference()    { if ( !--mRefCount ) delete this; }
@@ -63,6 +71,8 @@ class NS_COM nsSlidingSharedBufferList
 
     private:
       PRUint32 mRefCount;
+      nsFreeProc *mFreeProc;
+      void *mClientData;
   };
 
 
@@ -168,7 +178,12 @@ class NS_COM nsSlidingString
     public:
       nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd );
         // ...created by consuming ownership of a buffer ... |aStorageStart| must point to something
-        //  that it will be OK for the slidking string to call |nsMemory::Free| on
+        //  that it will be OK for the sliding string to call |nsMemory::Free| on
+
+      nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd,
+                       nsFreeProc *aFreeProc, void *aClientData = NULL);
+        // ...created by consuming ownership of a buffer ... |aStorageStart| must point to something
+        //  that it will be OK for the sliding string to call (*aFreeProc)(ptr, aClientData) on
 
       virtual PRUint32 Length() const { return mLength; }
 
diff --git a/mozilla/string/src/nsSlidingString.cpp b/mozilla/string/src/nsSlidingString.cpp
index 09e61c940d6..659bdb92cd8 100755
--- a/mozilla/string/src/nsSlidingString.cpp
+++ b/mozilla/string/src/nsSlidingString.cpp
@@ -41,8 +41,14 @@ nsSlidingSharedBufferList::DiscardUnreferencedPrefix( Buffer* aRecentlyReleasedB
   {
     if ( aRecentlyReleasedBuffer == mFirstBuffer )
       {
-        while ( mFirstBuffer && !mFirstBuffer->IsReferenced() )
-          delete UnlinkBuffer(mFirstBuffer);
+        while ( mFirstBuffer && !mFirstBuffer->IsReferenced() ) {
+          Buffer *buffer = UnlinkBuffer(mFirstBuffer);
+          if (mFreeProc && !buffer->IsSingleAllocationWithBuffer()) {
+            (*mFreeProc)(buffer->DataStart(), mClientData);
+            buffer->DataStart(nsnull);
+          }
+          delete buffer;
+        }
       }
   }
 
@@ -279,6 +285,13 @@ nsSlidingString::nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd,
     // nothing else to do here
   }
 
+nsSlidingString::nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd,
+                                  nsFreeProc *aFreeProc, void *aClientData )
+    : nsSlidingSubstring(new nsSlidingSharedBufferList(nsSlidingSharedBufferList::NewWrappingBuffer(aStorageStart, aDataEnd, aStorageEnd), aFreeProc, aClientData))
+  {
+    // nothing else to do here
+  }
+
 void
 nsSlidingString::AppendBuffer( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd )
   {
diff --git a/mozilla/xpcom/string/public/nsSharedBufferList.h b/mozilla/xpcom/string/public/nsSharedBufferList.h
index 335d982cd4d..5c160296280 100755
--- a/mozilla/xpcom/string/public/nsSharedBufferList.h
+++ b/mozilla/xpcom/string/public/nsSharedBufferList.h
@@ -91,6 +91,12 @@ class NS_COM nsSharedBufferList
                 mutable_this->set_refcount( get_refcount()-1 );
               }
 
+            PRBool
+            IsSingleAllocationWithBuffer() const
+              {
+                return (mFlags & kIsSingleAllocationWithBuffer);
+              }
+
             Buffer* mPrev;
             Buffer* mNext;
 
diff --git a/mozilla/xpcom/string/public/nsSlidingString.h b/mozilla/xpcom/string/public/nsSlidingString.h
index 5c4901319b0..0f7a34248a6 100755
--- a/mozilla/xpcom/string/public/nsSlidingString.h
+++ b/mozilla/xpcom/string/public/nsSlidingString.h
@@ -33,6 +33,11 @@
 #include "nsSharedBufferList.h"
 #endif
 
+/*
+ * nsFreeProc : A pointer to the free function used by nsSlidingString
+ */
+typedef void nsFreeProc(void *ptr, void *clientdata);
+
 
   /**
    * Maintains the sequence from the prev-most referenced buffer to the last buffer.
@@ -54,7 +59,10 @@ class NS_COM nsSlidingSharedBufferList
     : public nsSharedBufferList
   {
     public:
-      nsSlidingSharedBufferList( Buffer* aBuffer ) : nsSharedBufferList(aBuffer), mRefCount(0) { }
+      nsSlidingSharedBufferList( Buffer* aBuffer )
+          : nsSharedBufferList(aBuffer), mRefCount(0), mFreeProc(nsnull), mClientData(nsnull) { }
+      nsSlidingSharedBufferList( Buffer* aBuffer, nsFreeProc *aFreeproc, void *aClientData = NULL )
+          : nsSharedBufferList(aBuffer), mRefCount(0), mFreeProc(aFreeproc), mClientData(aClientData) { }
 
       void  AcquireReference()    { ++mRefCount; }
       void  ReleaseReference()    { if ( !--mRefCount ) delete this; }
@@ -63,6 +71,8 @@ class NS_COM nsSlidingSharedBufferList
 
     private:
       PRUint32 mRefCount;
+      nsFreeProc *mFreeProc;
+      void *mClientData;
   };
 
 
@@ -168,7 +178,12 @@ class NS_COM nsSlidingString
     public:
       nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd );
         // ...created by consuming ownership of a buffer ... |aStorageStart| must point to something
-        //  that it will be OK for the slidking string to call |nsMemory::Free| on
+        //  that it will be OK for the sliding string to call |nsMemory::Free| on
+
+      nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd,
+                       nsFreeProc *aFreeProc, void *aClientData = NULL);
+        // ...created by consuming ownership of a buffer ... |aStorageStart| must point to something
+        //  that it will be OK for the sliding string to call (*aFreeProc)(ptr, aClientData) on
 
       virtual PRUint32 Length() const { return mLength; }
 
diff --git a/mozilla/xpcom/string/src/nsSlidingString.cpp b/mozilla/xpcom/string/src/nsSlidingString.cpp
index 09e61c940d6..659bdb92cd8 100755
--- a/mozilla/xpcom/string/src/nsSlidingString.cpp
+++ b/mozilla/xpcom/string/src/nsSlidingString.cpp
@@ -41,8 +41,14 @@ nsSlidingSharedBufferList::DiscardUnreferencedPrefix( Buffer* aRecentlyReleasedB
   {
     if ( aRecentlyReleasedBuffer == mFirstBuffer )
       {
-        while ( mFirstBuffer && !mFirstBuffer->IsReferenced() )
-          delete UnlinkBuffer(mFirstBuffer);
+        while ( mFirstBuffer && !mFirstBuffer->IsReferenced() ) {
+          Buffer *buffer = UnlinkBuffer(mFirstBuffer);
+          if (mFreeProc && !buffer->IsSingleAllocationWithBuffer()) {
+            (*mFreeProc)(buffer->DataStart(), mClientData);
+            buffer->DataStart(nsnull);
+          }
+          delete buffer;
+        }
       }
   }
 
@@ -279,6 +285,13 @@ nsSlidingString::nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd,
     // nothing else to do here
   }
 
+nsSlidingString::nsSlidingString( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd,
+                                  nsFreeProc *aFreeProc, void *aClientData )
+    : nsSlidingSubstring(new nsSlidingSharedBufferList(nsSlidingSharedBufferList::NewWrappingBuffer(aStorageStart, aDataEnd, aStorageEnd), aFreeProc, aClientData))
+  {
+    // nothing else to do here
+  }
+
 void
 nsSlidingString::AppendBuffer( PRUnichar* aStorageStart, PRUnichar* aDataEnd, PRUnichar* aStorageEnd )
   {