diff --git a/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoFontScaleListener.java b/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoFontScaleListener.java
index 4c26bf7dd6f1..5704ec6d3225 100644
--- a/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoFontScaleListener.java
+++ b/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoFontScaleListener.java
@@ -156,6 +156,10 @@ import android.util.Log;
 
         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);
+            // Older Android versions don't sanitize the FONT_SCALE value. See Bug 1656078.
+            if (fontScale < 0) {
+                fontScale = DEFAULT_FONT_SCALE;
+            }
         } else { // We were turned off.
             fontScale = mPrevGeckoFontScale;
         }
diff --git a/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoRuntimeSettings.java b/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoRuntimeSettings.java
index 9d05b0b10e68..59ee4e9ca66e 100644
--- a/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoRuntimeSettings.java
+++ b/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoRuntimeSettings.java
@@ -23,6 +23,7 @@ import android.support.annotation.IntDef;
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 import android.text.TextUtils;
+import android.util.Log;
 
 import org.mozilla.gecko.EventDispatcher;
 import org.mozilla.gecko.GeckoSystemStateListener;
@@ -30,6 +31,8 @@ import org.mozilla.gecko.util.GeckoBundle;
 
 @AnyThread
 public final class GeckoRuntimeSettings extends RuntimeSettings {
+    private static final String LOGTAG = "GeckoRuntimeSettings";
+
     /**
      * Settings builder used to construct the settings object.
      */
@@ -917,13 +920,24 @@ public final class GeckoRuntimeSettings extends RuntimeSettings {
         return setFontSizeFactorInternal(fontSizeFactor);
     }
 
-    /* package */ @NonNull GeckoRuntimeSettings setFontSizeFactorInternal(
-            final float fontSizeFactor) {
+    private final static float DEFAULT_FONT_SIZE_FACTOR = 1f;
+
+    private float sanitizeFontSizeFactor(final float fontSizeFactor) {
         if (fontSizeFactor < 0) {
-            throw new IllegalArgumentException("fontSizeFactor cannot be < 0");
+            if (BuildConfig.DEBUG) {
+                throw new IllegalArgumentException("fontSizeFactor cannot be < 0");
+            } else {
+                Log.e(LOGTAG, "fontSizeFactor cannot be < 0");
+                return DEFAULT_FONT_SIZE_FACTOR;
+            }
         }
 
-        final int fontSizePercentage = Math.round(fontSizeFactor * 100);
+        return fontSizeFactor;
+    }
+
+    /* package */ @NonNull GeckoRuntimeSettings setFontSizeFactorInternal(
+            final float fontSizeFactor) {
+        final int fontSizePercentage = Math.round(sanitizeFontSizeFactor(fontSizeFactor) * 100);
         mFontSizeFactor.commit(fontSizePercentage);
         if (getFontInflationEnabled()) {
             final int scaledFontInflation = Math.round(FONT_INFLATION_BASE_VALUE * fontSizeFactor);