--- a/mobile/android/base/java/org/mozilla/gecko/GeckoFontScaleListener.java
+++ b/mobile/android/base/java/org/mozilla/gecko/GeckoFontScaleListener.java
@@ -103,25 +103,41 @@ class GeckoFontScaleListener
             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);
+        boolean useSystemFontScale;
+        if (prefs.contains(GeckoPreferences.PREFS_SYSTEM_FONT_SIZE)) {
+            useSystemFontScale = prefs.getBoolean(GeckoPreferences.PREFS_SYSTEM_FONT_SIZE, false);
+        } else {
+            migrateOldSetting(prefs);
+            return;
+        }
 
         if (useSystemFontScale) {
             start();
         } else {
             stop();
         }
     }
 
+    private void migrateOldSetting(final SharedPreferences prefs) {
+        PrefsHelper.getPref(PREF_FONT_INFLATION, new PrefsHelper.PrefHandlerBase() {
+            @Override
+            public void prefValue(String pref, int value) {
+                // The first setting step above "Tiny" used to correspond to a value of 80.
+                prefs.edit().putBoolean(GeckoPreferences.PREFS_SYSTEM_FONT_SIZE, (value > 60)).apply();
+            }
+        });
+    }
+
     @Override
     public void onChange(boolean selfChange) {
         onSystemFontScaleChange(mApplicationContext.getContentResolver(), false);
     }
 
     @UiThread // According to the docs.
     @Override
     public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {