diff --git a/modules/libpref/src/nsPref.cpp b/modules/libpref/src/nsPref.cpp
index 88c01cedc66..f82dd8945f0 100644
--- a/modules/libpref/src/nsPref.cpp
+++ b/modules/libpref/src/nsPref.cpp
@@ -48,6 +48,7 @@
 #include "nsISecurityPref.h"
 #include "nsIPrefService.h"
 #include "nsISupportsPrimitives.h"
+#include "nsWeakReference.h"
 
 #include "nsIJSRuntimeService.h"
 #include "nsIModule.h"
@@ -63,7 +64,8 @@ class nsPref : public nsIPref,
                public nsIObserver,
                public nsIPrefBranch,
                public nsIPrefBranchInternal,
-               public nsISecurityPref
+               public nsISecurityPref,
+               public nsSupportsWeakReference
 {
 public:
   static nsPref *GetInstance();
@@ -124,7 +126,7 @@ nsPref* nsPref::gInstance = NULL;
 static PRInt32 g_InstanceCount = 0;
 
 
-NS_IMPL_THREADSAFE_ISUPPORTS6(nsPref, nsIPref, nsIPrefService, nsIObserver, nsIPrefBranch, nsIPrefBranchInternal, nsISecurityPref);
+NS_IMPL_THREADSAFE_ISUPPORTS7(nsPref, nsIPref, nsIPrefService, nsIObserver, nsIPrefBranch, nsIPrefBranchInternal, nsISecurityPref, nsISupportsWeakReference);
 
 //----------------------------------------------------------------------------------------
 nsPref::nsPref()
diff --git a/modules/libpref/src/nsPrefBranch.cpp b/modules/libpref/src/nsPrefBranch.cpp
index 2f758b51267..dbc4d057938 100644
--- a/modules/libpref/src/nsPrefBranch.cpp
+++ b/modules/libpref/src/nsPrefBranch.cpp
@@ -142,6 +142,7 @@ NS_INTERFACE_MAP_BEGIN(nsPrefBranch)
   NS_INTERFACE_MAP_ENTRY(nsIPrefBranch)
   NS_INTERFACE_MAP_ENTRY(nsIPrefBranchInternal)
   NS_INTERFACE_MAP_ENTRY(nsISecurityPref)
+  NS_INTERFACE_MAP_ENTRY(nsISupportsWeakReference)
 NS_INTERFACE_MAP_END
 
 
diff --git a/modules/libpref/src/nsPrefBranch.h b/modules/libpref/src/nsPrefBranch.h
index b7caf005fd8..378fd78f7f0 100644
--- a/modules/libpref/src/nsPrefBranch.h
+++ b/modules/libpref/src/nsPrefBranch.h
@@ -31,10 +31,12 @@
 #include "nsISupportsPrimitives.h"
 #include "nsString.h"
 #include "nsVoidArray.h"
+#include "nsWeakReference.h"
 
 class nsPrefBranch : public nsIPrefBranch,
                      public nsIPrefBranchInternal,
-                     public nsISecurityPref
+                     public nsISecurityPref,
+                     public nsSupportsWeakReference
 {
 public:
   NS_DECL_ISUPPORTS
diff --git a/modules/libpref/src/nsPrefService.cpp b/modules/libpref/src/nsPrefService.cpp
index a6fd7bbdb76..0e6a128e42d 100644
--- a/modules/libpref/src/nsPrefService.cpp
+++ b/modules/libpref/src/nsPrefService.cpp
@@ -97,6 +97,7 @@ NS_INTERFACE_MAP_BEGIN(nsPrefService)
     NS_INTERFACE_MAP_ENTRY(nsIObserver)
     NS_INTERFACE_MAP_ENTRY(nsIPrefBranch)
     NS_INTERFACE_MAP_ENTRY(nsIPrefBranchInternal)
+    NS_INTERFACE_MAP_ENTRY(nsISupportsWeakReference)
 NS_INTERFACE_MAP_END
 
 
diff --git a/modules/libpref/src/nsPrefService.h b/modules/libpref/src/nsPrefService.h
index 7260f9217a2..12981298754 100644
--- a/modules/libpref/src/nsPrefService.h
+++ b/modules/libpref/src/nsPrefService.h
@@ -26,13 +26,15 @@
 #include "nsIPrefBranchInternal.h"
 #include "nsIObserver.h"
 #include "nsCOMPtr.h"
+#include "nsWeakReference.h"
 
 class nsIFile;
 
 class nsPrefService : public nsIPrefService,
                       public nsIObserver,
                       public nsIPrefBranch,
-                      public nsIPrefBranchInternal
+                      public nsIPrefBranchInternal,
+                      public nsSupportsWeakReference
 {
 public:
   NS_DECL_ISUPPORTS