gecko-dev/mobile/android/base/resources/drawable/menu_level.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- 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/. -->

<level-list xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:gecko="http://schemas.android.com/apk/res-auto">

    <item android:maxLevel="1">

        <selector>

            <item gecko:state_private="true" android:drawable="@drawable/menu_pb"/>
            <item android:drawable="@drawable/menu"/>

        </selector>

    </item>

    <item android:maxLevel="2" android:drawable="@android:color/transparent"/>

</level-list>