--- a/gfx/thebes/gfxPlatform.cpp
+++ b/gfx/thebes/gfxPlatform.cpp
@@ -448,39 +448,22 @@ SRGBOverrideObserver::Observe(nsISupport
 
 static const char* kObservedPrefs[] = {
     "gfx.downloadable_fonts.",
     "gfx.font_rendering.",
     BIDI_NUMERAL_PREF,
     nullptr
 };
 
-class FontPrefsObserver final : public nsIObserver
+static void
+FontPrefChanged(const char* aPref, void* aData)
 {
-    ~FontPrefsObserver() = default;
-public:
-    NS_DECL_ISUPPORTS
-    NS_DECL_NSIOBSERVER
-};
-
-NS_IMPL_ISUPPORTS(FontPrefsObserver, nsIObserver)
-
-NS_IMETHODIMP
-FontPrefsObserver::Observe(nsISupports *aSubject,
-                           const char *aTopic,
-                           const char16_t *someData)
-{
-    if (!someData) {
-        NS_ERROR("font pref observer code broken");
-        return NS_ERROR_UNEXPECTED;
-    }
+    MOZ_ASSERT(aPref);
     NS_ASSERTION(gfxPlatform::GetPlatform(), "the singleton instance has gone");
-    gfxPlatform::GetPlatform()->FontsPrefsChanged(NS_ConvertUTF16toUTF8(someData).get());
-
-    return NS_OK;
+    gfxPlatform::GetPlatform()->FontsPrefsChanged(aPref);
 }
 
 class MemoryPressureObserver final : public nsIObserver
 {
     ~MemoryPressureObserver() = default;
 public:
     NS_DECL_ISUPPORTS
     NS_DECL_NSIOBSERVER
@@ -818,18 +801,17 @@ gfxPlatform::Init()
     if (NS_FAILED(rv)) {
         MOZ_CRASH("Could not initialize gfxFontCache");
     }
 
     /* Create and register our CMS Override observer. */
     gPlatform->mSRGBOverrideObserver = new SRGBOverrideObserver();
     Preferences::AddWeakObserver(gPlatform->mSRGBOverrideObserver, GFX_PREF_CMS_FORCE_SRGB);
 
-    gPlatform->mFontPrefsObserver = new FontPrefsObserver();
-    Preferences::AddStrongObservers(gPlatform->mFontPrefsObserver, kObservedPrefs);
+    Preferences::RegisterPrefixCallbacks(FontPrefChanged, kObservedPrefs);
 
     GLContext::PlatformStartup();
 
     Preferences::RegisterCallbackAndCall(RecordingPrefChanged, "gfx.2d.recording");
 
     CreateCMSOutputProfile();
 
     // Listen to memory pressure event so we can purge DrawTarget caches
@@ -993,19 +975,17 @@ gfxPlatform::Shutdown()
     // Free the various non-null transforms and loaded profiles
     ShutdownCMS();
 
     /* Unregister our CMS Override callback. */
     NS_ASSERTION(gPlatform->mSRGBOverrideObserver, "mSRGBOverrideObserver has alreay gone");
     Preferences::RemoveObserver(gPlatform->mSRGBOverrideObserver, GFX_PREF_CMS_FORCE_SRGB);
     gPlatform->mSRGBOverrideObserver = nullptr;
 
-    NS_ASSERTION(gPlatform->mFontPrefsObserver, "mFontPrefsObserver has alreay gone");
-    Preferences::RemoveObservers(gPlatform->mFontPrefsObserver, kObservedPrefs);
-    gPlatform->mFontPrefsObserver = nullptr;
+    Preferences::UnregisterPrefixCallbacks(FontPrefChanged, kObservedPrefs);
 
     NS_ASSERTION(gPlatform->mMemoryPressureObserver, "mMemoryPressureObserver has already gone");
     nsCOMPtr<nsIObserverService> obs = mozilla::services::GetObserverService();
     if (obs) {
         obs->RemoveObserver(gPlatform->mMemoryPressureObserver, "memory-pressure");
     }
 
     gPlatform->mMemoryPressureObserver = nullptr;

--- a/gfx/thebes/gfxPlatform.h
+++ b/gfx/thebes/gfxPlatform.h
@@ -883,17 +883,16 @@ private:
     void InitGPUProcessPrefs();
     void InitOMTPConfig();
 
     static bool IsDXInterop2Blocked();
     static bool IsDXNV12Blocked();
 
     RefPtr<gfxASurface> mScreenReferenceSurface;
     nsCOMPtr<nsIObserver> mSRGBOverrideObserver;
-    nsCOMPtr<nsIObserver> mFontPrefsObserver;
     nsCOMPtr<nsIObserver> mMemoryPressureObserver;
 
     // The preferred draw target backend to use for canvas
     mozilla::gfx::BackendType mPreferredCanvasBackend;
     // The fallback draw target backend to use for canvas, if the preferred backend fails
     mozilla::gfx::BackendType mFallbackCanvasBackend;
     // The backend to use for content
     mozilla::gfx::BackendType mContentBackend;