gecko-dev/mobile/android/base/resources/drawable/tabs_button.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">

    <item android:maxLevel="1">
        <selector>
            <item android:state_pressed="true" android:drawable="@drawable/highlight"/>
            <item android:drawable="@drawable/tabs_tray_bg_repeat"/>
        </selector>
    </item>

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

</level-list>