$this->context->smarty->assign(array(
    'mobile_device' => $this->context->getMobileDevice(),

if (!isset($this->context->cookie->mob))
{
    require_once(_PS_TOOL_DIR_.'mobile_Detect/Mobile_Detect.php');
    $mobile_detect = new Mobile_Detect();
    if ($mobile_detect->isMobile() || $mobile_detect->isTablet())
        $this->context->cookie->mob = 1;
    else
        $this->context->cookie->mob = 0;
}

'mobile_device' => $this->context->getMobileDevice(),

'mobile_device' => $this->context->cookie->mob,

{if $mobile_device}
    ...
{/if}