--- a/mobile/android/base/java/org/mozilla/gecko/GeckoApplication.java
+++ b/mobile/android/base/java/org/mozilla/gecko/GeckoApplication.java
@@ -125,16 +125,17 @@ public class GeckoApplication extends Ap
 
             GeckoNetworkManager.getInstance().stop();
         }
     }
 
     public void onActivityResume(GeckoActivityStatus activity) {
         if (mIsInitialResume) {
             GeckoBatteryManager.getInstance().start(this);
+            GeckoFontScaleListener.getInstance().initialize(this);
             GeckoNetworkManager.getInstance().start(this);
             mIsInitialResume = false;
         } else if (mPausedGecko) {
             GeckoThread.onResume();
             mPausedGecko = false;
             GeckoNetworkManager.getInstance().start(this);
         }
 

new file mode 100644
--- /dev/null
+++ b/mobile/android/base/java/org/mozilla/gecko/GeckoFontScaleListener.java
@@ -0,0 +1,134 @@
+/* -*- 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 android.content.ContentResolver;
+import android.content.Context;
+import android.database.ContentObserver;
+import android.content.SharedPreferences;
+import android.net.Uri;
+import android.provider.Settings;
+import android.support.annotation.UiThread;
+import android.util.Log;
+
+import org.mozilla.gecko.preferences.GeckoPreferences;
+
+class GeckoFontScaleListener
+        extends ContentObserver
+        implements SharedPreferences.OnSharedPreferenceChangeListener {
+    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;
+
+    private static final GeckoFontScaleListener listenerInstance = new GeckoFontScaleListener();
+
+    private Context applicationContext;
+    private boolean initialized;
+    private boolean running;
+
+    public static GeckoFontScaleListener getInstance() {
+        return listenerInstance;
+    }
+
+    private GeckoFontScaleListener() {
+        super(null);
+    }
+
+    public synchronized void initialize(final Context context) {
+        if (initialized) {
+            Log.w(LOGTAG, "Already initialized!");
+            return;
+        }
+
+        applicationContext = context.getApplicationContext();
+        SharedPreferences prefs = GeckoSharedPrefs.forApp(applicationContext);
+        prefs.registerOnSharedPreferenceChangeListener(this);
+        onPrefChange(prefs);
+        initialized = true;
+    }
+
+    public synchronized void shutdown() {
+        if (!initialized) {
+            Log.w(LOGTAG, "Already shut down!");
+            return;
+        }
+
+        GeckoSharedPrefs.forApp(applicationContext).unregisterOnSharedPreferenceChangeListener(this);
+        stop();
+        applicationContext = null;
+        initialized = false;
+    }
+
+    private synchronized void start() {
+        if (running) {
+            return;
+        }
+
+        ContentResolver contentResolver = applicationContext.getContentResolver();
+        Uri fontSizeSetting = Settings.System.getUriFor(Settings.System.FONT_SCALE);
+        contentResolver.registerContentObserver(fontSizeSetting, false, this);
+        onSystemFontScaleChange(contentResolver, false);
+
+        running = true;
+    }
+
+    private synchronized void stop() {
+        if (!running) {
+            return;
+        }
+
+        ContentResolver contentResolver = applicationContext.getContentResolver();
+        contentResolver.unregisterContentObserver(this);
+        onSystemFontScaleChange(contentResolver, /*stopping*/ true);
+
+        running = 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.
+            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".
+            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();
+        }
+    }
+
+    @Override
+    public void onChange(boolean selfChange) {
+        onSystemFontScaleChange(applicationContext.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);
+    }
+}

--- a/mobile/android/base/moz.build
+++ b/mobile/android/base/moz.build
@@ -449,16 +449,17 @@ gbjar.sources += ['java/org/mozilla/geck
     'firstrun/RestrictedWelcomePanel.java',
     'firstrun/SyncPanel.java',
     'firstrun/TabQueuePanel.java',
     'FormAssistPopup.java',
     'GeckoActivity.java',
     'GeckoActivityStatus.java',
     'GeckoApp.java',
     'GeckoApplication.java',
+    'GeckoFontScaleListener.java',
     'GeckoJavaSampler.java',
     'GeckoMessageReceiver.java',
     'GeckoProfilesProvider.java',
     'GeckoService.java',
     'GeckoUpdateReceiver.java',
     'GlobalHistory.java',
     'GlobalPageMetadata.java',
     'GuestSession.java',