--- a/mobile/android/base/java/org/mozilla/gecko/GeckoApplication.java
+++ b/mobile/android/base/java/org/mozilla/gecko/GeckoApplication.java
@@ -6,16 +6,17 @@ package org.mozilla.gecko;
 
 import android.Manifest;
 import android.app.Activity;
 import android.app.Application;
 import android.app.Service;
 import android.content.ContentResolver;
 import android.content.Context;
 import android.content.Intent;
+import android.content.SharedPreferences;
 import android.content.res.Configuration;
 import android.graphics.Bitmap;
 import android.net.Uri;
 import android.os.Bundle;
 import android.os.Environment;
 import android.os.Parcelable;
 import android.os.Process;
 import android.os.SystemClock;
@@ -44,16 +45,17 @@ import org.mozilla.gecko.icons.Icons;
 import org.mozilla.gecko.lwt.LightweightTheme;
 import org.mozilla.gecko.mdns.MulticastDNSManager;
 import org.mozilla.gecko.media.AudioFocusAgent;
 import org.mozilla.gecko.mozglue.SafeIntent;
 import org.mozilla.gecko.notifications.NotificationClient;
 import org.mozilla.gecko.notifications.NotificationHelper;
 import org.mozilla.gecko.permissions.Permissions;
 import org.mozilla.gecko.preferences.DistroSharedPrefsImport;
+import org.mozilla.gecko.preferences.GeckoPreferences;
 import org.mozilla.gecko.pwa.PwaUtils;
 import org.mozilla.gecko.telemetry.TelemetryBackgroundReceiver;
 import org.mozilla.gecko.util.ActivityResultHandler;
 import org.mozilla.gecko.util.BitmapUtils;
 import org.mozilla.gecko.util.BundleEventListener;
 import org.mozilla.gecko.util.EventCallback;
 import org.mozilla.gecko.util.GeckoBundle;
 import org.mozilla.gecko.util.HardwareUtils;
@@ -67,17 +69,18 @@ import java.io.ByteArrayOutputStream;
 import java.io.File;
 import java.io.InputStream;
 import java.io.IOException;
 import java.lang.reflect.Method;
 import java.net.URL;
 import java.util.UUID;
 
 public class GeckoApplication extends Application
-                              implements HapticFeedbackDelegate {
+                              implements HapticFeedbackDelegate,
+                                         SharedPreferences.OnSharedPreferenceChangeListener {
     private static final String LOG_TAG = "GeckoApplication";
     public static final String ACTION_DEBUG = "org.mozilla.gecko.DEBUG";
     private static final String MEDIA_DECODING_PROCESS_CRASH = "MEDIA_DECODING_PROCESS_CRASH";
 
     private boolean mInBackground;
     private boolean mPausedGecko;
     private boolean mIsInitialResume;
 
@@ -207,28 +210,35 @@ public class GeckoApplication extends Ap
         });
 
         GeckoNetworkManager.getInstance().stop();
     }
 
     public void onApplicationForeground() {
         if (mIsInitialResume) {
             GeckoBatteryManager.getInstance().start(this);
-            GeckoFontScaleListener.getInstance().attachToContext(this);
+            initFontScaleListener();
             GeckoNetworkManager.getInstance().start(this);
             mIsInitialResume = false;
         } else if (mPausedGecko) {
             GeckoThread.onResume();
             mPausedGecko = false;
             GeckoNetworkManager.getInstance().start(this);
         }
 
         mInBackground = false;
     }
 
+    private void initFontScaleListener() {
+        final SharedPreferences prefs = GeckoSharedPrefs.forApp(this);
+        prefs.registerOnSharedPreferenceChangeListener(this);
+        onSharedPreferenceChanged(prefs, GeckoPreferences.PREFS_SYSTEM_FONT_SIZE);
+        GeckoFontScaleListener.getInstance().attachToContext(this);
+    }
+
     private static GeckoRuntime sGeckoRuntime;
     public static GeckoRuntime getRuntime() {
         return sGeckoRuntime;
     }
 
     public static GeckoRuntime ensureRuntime(@NonNull Context context) {
         if (sGeckoRuntime != null) {
             return sGeckoRuntime;
@@ -417,16 +427,17 @@ public class GeckoApplication extends Ap
                 "Share:Text",
                 null);
         EventDispatcher.getInstance().unregisterBackgroundThreadListener(mListener,
                 "PushServiceAndroidGCM:Configure",
                 "Bookmark:Insert",
                 "Image:SetAs",
                 "Profile:Create",
                 null);
+        GeckoSharedPrefs.forApp(this).unregisterOnSharedPreferenceChangeListener(this);
 
         GeckoService.unregister();
     }
 
     /* package */ boolean initPushService() {
         // It's fine to throw GCM initialization onto a background thread; the registration process requires
         // network access, so is naturally asynchronous.  This, of course, races against Gecko page load of
         // content requiring GCM-backed services, like Web Push.  There's nothing to be done here.
@@ -904,9 +915,17 @@ public class GeckoApplication extends Ap
     @Override // HapticFeedbackDelegate
     public void performHapticFeedback(final int effect) {
         final Activity currentActivity =
                 GeckoActivityMonitor.getInstance().getCurrentActivity();
         if (currentActivity != null) {
             currentActivity.getWindow().getDecorView().performHapticFeedback(effect);
         }
     }
+
+    @Override // OnSharedPreferenceChangeListener
+    public void onSharedPreferenceChanged(SharedPreferences prefs, String key) {
+        if (GeckoPreferences.PREFS_SYSTEM_FONT_SIZE.equals(key)) {
+            final boolean enabled = prefs.getBoolean(GeckoPreferences.PREFS_SYSTEM_FONT_SIZE, false);
+            GeckoFontScaleListener.getInstance().setEnabled(enabled);
+        }
+    }
 }

--- a/mobile/android/base/java/org/mozilla/gecko/GeckoFontScaleListener.java
+++ b/mobile/android/base/java/org/mozilla/gecko/GeckoFontScaleListener.java
@@ -1,45 +1,43 @@
 /* -*- Mode: Java; c-basic-offset: 4; tab-width: 4; indent-tabs-mode: nil; -*-
  * This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 package org.mozilla.gecko;
 
-import org.mozilla.gecko.preferences.GeckoPreferences;
 import org.mozilla.gecko.util.ThreadUtils;
 
 import android.annotation.SuppressLint;
 import android.content.ContentResolver;
 import android.content.Context;
-import android.content.SharedPreferences;
 import android.database.ContentObserver;
 import android.net.Uri;
 import android.provider.Settings;
 import android.support.annotation.UiThread;
 import android.util.Log;
 
 class GeckoFontScaleListener
-        extends ContentObserver
-        implements SharedPreferences.OnSharedPreferenceChangeListener {
+        extends ContentObserver {
     private static final String LOGTAG = "GeckoFontScaleListener";
 
     private static final String PREF_SYSTEM_FONT_SCALE = "font.size.systemFontScale";
     private static final String PREF_FONT_INFLATION = "font.size.inflation.minTwips";
     private static final int FONT_INFLATION_OFF = 0;
     private static final int FONT_INFLATION_ON_DEFAULT_VALUE = 120;
     private static final float DEFAULT_FONT_SCALE = 1.0f;
 
     // We're referencing the *application* context, so this is in fact okay.
     @SuppressLint("StaticFieldLeak")
     private static final GeckoFontScaleListener sInstance = new GeckoFontScaleListener();
 
     private Context mApplicationContext;
     private boolean mAttached;
+    private boolean mEnabled;
     private boolean mRunning;
 
     public static GeckoFontScaleListener getInstance() {
         return sInstance;
     }
 
     private GeckoFontScaleListener() {
         // Ensure the ContentObserver callback runs on the UI thread.
@@ -49,37 +47,56 @@ class GeckoFontScaleListener
     public void attachToContext(final Context context) {
         ThreadUtils.assertOnUiThread();
 
         if (mAttached) {
             Log.w(LOGTAG, "Already attached!");
             return;
         }
 
+        mAttached = true;
         mApplicationContext = context.getApplicationContext();
-        SharedPreferences prefs = GeckoSharedPrefs.forApp(mApplicationContext);
-        prefs.registerOnSharedPreferenceChangeListener(this);
-        onPrefChange(prefs);
-        mAttached = true;
+        onEnabledChange();
     }
 
     public void detachFromContext() {
         ThreadUtils.assertOnUiThread();
 
         if (!mAttached) {
             Log.w(LOGTAG, "Already detached!");
             return;
         }
 
-        GeckoSharedPrefs.forApp(mApplicationContext).unregisterOnSharedPreferenceChangeListener(this);
         stop();
         mApplicationContext = null;
         mAttached = false;
     }
 
+    public void setEnabled(boolean enabled) {
+        ThreadUtils.assertOnUiThread();
+        mEnabled = enabled;
+        onEnabledChange();
+    }
+
+    public boolean getEnabled() {
+        return mEnabled;
+    }
+
+    private void onEnabledChange() {
+        if (!mAttached) {
+            return;
+        }
+
+        if (mEnabled) {
+            start();
+        } else {
+            stop();
+        }
+    }
+
     private void start() {
         if (mRunning) {
             return;
         }
 
         ContentResolver contentResolver = mApplicationContext.getContentResolver();
         Uri fontSizeSetting = Settings.System.getUriFor(Settings.System.FONT_SCALE);
         contentResolver.registerContentObserver(fontSizeSetting, false, this);
@@ -99,46 +116,26 @@ class GeckoFontScaleListener
 
         mRunning = false;
     }
 
     private void onSystemFontScaleChange(final ContentResolver contentResolver, boolean stopping) {
         float fontScale;
         int fontInflation;
 
-        if (!stopping) { // Pref was flipped to "On" or system font scale changed.
+        if (!stopping) { // Either we were enabled, or else the system font scale changed.
             fontScale = Settings.System.getFloat(contentResolver, Settings.System.FONT_SCALE, DEFAULT_FONT_SCALE);
             fontInflation = Math.round(FONT_INFLATION_ON_DEFAULT_VALUE * fontScale);
-        } else { // Pref was flipped to "Off".
+        } else { // We were turned off.
             fontScale = DEFAULT_FONT_SCALE;
             fontInflation = FONT_INFLATION_OFF;
         }
 
         PrefsHelper.setPref(PREF_FONT_INFLATION, fontInflation);
         PrefsHelper.setPref(PREF_SYSTEM_FONT_SCALE, Math.round(fontScale * 100));
     }
 
-    private void onPrefChange(final SharedPreferences prefs) {
-        boolean useSystemFontScale = prefs.getBoolean(GeckoPreferences.PREFS_SYSTEM_FONT_SIZE, false);
-
-        if (useSystemFontScale) {
-            start();
-        } else {
-            stop();
-        }
-    }
-
     @UiThread // See constructor.
     @Override
     public void onChange(boolean selfChange) {
         onSystemFontScaleChange(mApplicationContext.getContentResolver(), false);
     }
-
-    @UiThread // According to the docs.
-    @Override
-    public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
-        if (!GeckoPreferences.PREFS_SYSTEM_FONT_SIZE.equals(key)) {
-            return;
-        }
-
-        onPrefChange(sharedPreferences);
-    }
 }