--- a/dom/base/nsFocusManager.cpp
+++ b/dom/base/nsFocusManager.cpp
@@ -183,17 +183,19 @@ static const char* kObservedPrefs[] = {
 };
 
 nsFocusManager::nsFocusManager()
   : mEventHandlingNeedsFlush(false)
 { }
 
 nsFocusManager::~nsFocusManager()
 {
-  Preferences::RemoveObservers(this, kObservedPrefs);
+  Preferences::UnregisterCallbacks(
+    PREF_CHANGE_METHOD(nsFocusManager::PrefChanged),
+    kObservedPrefs, this);
 
   nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
   if (obs) {
     obs->RemoveObserver(this, "xpcom-shutdown");
   }
 }
 
 // static
@@ -208,57 +210,63 @@ nsFocusManager::Init()
     Preferences::GetBool("accessibility.tabfocus_applies_to_xul",
                          nsIContent::sTabFocusModelAppliesToXUL);
 
   sMouseFocusesFormControl =
     Preferences::GetBool("accessibility.mouse_focuses_formcontrol", false);
 
   sTestMode = Preferences::GetBool("focusmanager.testmode", false);
 
-  Preferences::AddWeakObservers(fm, kObservedPrefs);
+  Preferences::RegisterCallbacks(
+    PREF_CHANGE_METHOD(nsFocusManager::PrefChanged),
+    kObservedPrefs, fm);
 
   nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
   if (obs) {
     obs->AddObserver(fm, "xpcom-shutdown", true);
   }
 
   return NS_OK;
 }
 
 // static
 void
 nsFocusManager::Shutdown()
 {
   NS_IF_RELEASE(sInstance);
 }
 
+void
+nsFocusManager::PrefChanged(const char* aPref)
+{
+  nsDependentCString pref(aPref);
+  if (pref.EqualsLiteral("accessibility.browsewithcaret")) {
+    UpdateCaretForCaretBrowsingMode();
+  }
+  else if (pref.EqualsLiteral("accessibility.tabfocus_applies_to_xul")) {
+    nsIContent::sTabFocusModelAppliesToXUL =
+      Preferences::GetBool("accessibility.tabfocus_applies_to_xul",
+                           nsIContent::sTabFocusModelAppliesToXUL);
+  }
+  else if (pref.EqualsLiteral("accessibility.mouse_focuses_formcontrol")) {
+    sMouseFocusesFormControl =
+      Preferences::GetBool("accessibility.mouse_focuses_formcontrol",
+                           false);
+  }
+  else if (pref.EqualsLiteral("focusmanager.testmode")) {
+    sTestMode = Preferences::GetBool("focusmanager.testmode", false);
+  }
+}
+
 NS_IMETHODIMP
 nsFocusManager::Observe(nsISupports *aSubject,
                         const char *aTopic,
                         const char16_t *aData)
 {
-  if (!nsCRT::strcmp(aTopic, NS_PREFBRANCH_PREFCHANGE_TOPIC_ID)) {
-    nsDependentString data(aData);
-    if (data.EqualsLiteral("accessibility.browsewithcaret")) {
-      UpdateCaretForCaretBrowsingMode();
-    }
-    else if (data.EqualsLiteral("accessibility.tabfocus_applies_to_xul")) {
-      nsIContent::sTabFocusModelAppliesToXUL =
-        Preferences::GetBool("accessibility.tabfocus_applies_to_xul",
-                             nsIContent::sTabFocusModelAppliesToXUL);
-    }
-    else if (data.EqualsLiteral("accessibility.mouse_focuses_formcontrol")) {
-      sMouseFocusesFormControl =
-        Preferences::GetBool("accessibility.mouse_focuses_formcontrol",
-                             false);
-    }
-    else if (data.EqualsLiteral("focusmanager.testmode")) {
-      sTestMode = Preferences::GetBool("focusmanager.testmode", false);
-    }
-  } else if (!nsCRT::strcmp(aTopic, "xpcom-shutdown")) {
+  if (!nsCRT::strcmp(aTopic, "xpcom-shutdown")) {
     mActiveWindow = nullptr;
     mFocusedWindow = nullptr;
     mFocusedElement = nullptr;
     mFirstBlurEvent = nullptr;
     mFirstFocusEvent = nullptr;
     mWindowBeingLowered = nullptr;
     mDelayedBlurFocusEvents.Clear();
     mMouseButtonEventHandlingDocument = nullptr;

--- a/dom/base/nsFocusManager.h
+++ b/dom/base/nsFocusManager.h
@@ -52,16 +52,18 @@ public:
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
   NS_DECL_NSIOBSERVER
   NS_DECL_NSIFOCUSMANAGER
 
   // called to initialize and stop the focus manager at startup and shutdown
   static nsresult Init();
   static void Shutdown();
 
+  void PrefChanged(const char* aPref);
+
   /**
    * Retrieve the single focus manager.
    */
   static nsFocusManager* GetFocusManager() { return sInstance; }
 
   /**
    * A faster version of nsIFocusManager::GetFocusedElement, returning a
    * raw Element pointer (instead of having AddRef-ed Element