Fix for "Flash Of Unstyled Content" and slow loading in the Firefox (#2913)

app/layout/layout.phtml

@@ -9,6 +9,7 @@ if (_t('gen.dir') === 'rtl') {
 		<meta charset="UTF-8" />
 		<meta name="viewport" content="initial-scale=1.0" />
 		<?= self::headStyle() ?>
+		<script>/* Firefox FOUC Fix */</script>
 		<script id="jsonVars" type="application/json">
 <?php $this->renderHelper('javascript_vars'); ?>
 		</script>