diff --git a/modules/libpref/public/Preferences.h b/modules/libpref/public/Preferences.h
index 37a57df3c02..751e8b23126 100644
--- a/modules/libpref/public/Preferences.h
+++ b/modules/libpref/public/Preferences.h
@@ -50,7 +50,6 @@ public:
   NS_DECL_NSIOBSERVER
 
   Preferences();
-  virtual ~Preferences();
 
   nsresult Init();
 
@@ -353,6 +352,8 @@ public:
   static nsresult SetFloat(const char* aPref, float aValue);
 
 protected:
+  virtual ~Preferences();
+
   nsresult NotifyServiceObservers(const char *aSubject);
   /**
    * Reads the default pref file or, if that failed, try to save a new one.
diff --git a/modules/libpref/src/Preferences.cpp b/modules/libpref/src/Preferences.cpp
index 9b48912c9ad..aa419e708d1 100644
--- a/modules/libpref/src/Preferences.cpp
+++ b/modules/libpref/src/Preferences.cpp
@@ -128,6 +128,10 @@ public:
 class ValueObserver MOZ_FINAL : public nsIObserver,
                                 public ValueObserverHashKey
 {
+  ~ValueObserver() {
+    Preferences::RemoveObserver(this, mPrefName.get());
+  }
+
 public:
   NS_DECL_ISUPPORTS
   NS_DECL_NSIOBSERVER
@@ -135,10 +139,6 @@ public:
   ValueObserver(const char *aPref, PrefChangedFunc aCallback)
     : ValueObserverHashKey(aPref, aCallback) { }
 
-  ~ValueObserver() {
-    Preferences::RemoveObserver(this, mPrefName.get());
-  }
-
   void AppendClosure(void *aClosure) {
     mClosures.AppendElement(aClosure);
   }
@@ -229,6 +229,8 @@ Preferences::SizeOfIncludingThisAndOtherStuff(mozilla::MallocSizeOf aMallocSizeO
 
 class PreferenceServiceReporter MOZ_FINAL : public nsIMemoryReporter
 {
+  ~PreferenceServiceReporter() {}
+
 public:
   NS_DECL_ISUPPORTS
   NS_DECL_NSIMEMORYREPORTER
diff --git a/modules/libpref/src/nsPrefBranch.h b/modules/libpref/src/nsPrefBranch.h
index 9db210df563..ed629e68630 100644
--- a/modules/libpref/src/nsPrefBranch.h
+++ b/modules/libpref/src/nsPrefBranch.h
@@ -187,7 +187,6 @@ public:
   NS_DECL_NSIOBSERVER
 
   nsPrefBranch(const char *aPrefRoot, bool aDefaultBranch);
-  virtual ~nsPrefBranch();
 
   int32_t GetRootLength() { return mPrefRootLength; }
 
@@ -198,6 +197,8 @@ public:
   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf);
 
 protected:
+  virtual ~nsPrefBranch();
+
   nsPrefBranch()    /* disallow use of this constructer */
     { }
 
@@ -232,7 +233,6 @@ class nsPrefLocalizedString : public nsIPrefLocalizedString,
 {
 public:
   nsPrefLocalizedString();
-  virtual ~nsPrefLocalizedString();
 
   NS_DECL_ISUPPORTS
   NS_FORWARD_NSISUPPORTSSTRING(mUnicodeString->)
@@ -241,6 +241,8 @@ public:
   nsresult Init();
 
 private:
+  virtual ~nsPrefLocalizedString();
+
   NS_IMETHOD GetData(char16_t**);
   NS_IMETHOD SetData(const char16_t* aData);
   NS_IMETHOD SetDataWithLength(uint32_t aLength, const char16_t *aData);
@@ -254,11 +256,12 @@ class nsRelativeFilePref : public nsIRelativeFilePref
 public:
   NS_DECL_ISUPPORTS
   NS_DECL_NSIRELATIVEFILEPREF
-  
-                nsRelativeFilePref();
-  virtual       ~nsRelativeFilePref();
-  
+
+  nsRelativeFilePref();
+
 private:
+  virtual ~nsRelativeFilePref();
+
   nsCOMPtr<nsIFile> mFile;
   nsCString mRelativeToKey;
 };