Magento – How to use customer_logout event in magento

To my understanding controller_front_init_before is not the best way for setting a cookie as you can miss some redirects and also you have a minimal data in your disposal.

I would recommend using controller_front_send_response_before as the last event triggered before output is sent to browser.

update your code with below code and try

<?php
namespace Vendor\Module\Observer;
use Magento\Framework\Event\Observer as EventObserver;
use Magento\Framework\Event\ObserverInterface;
class CustomereditPost implements ObserverInterface
{
protected $_request;
protected $_layout;
protected $_objectManager = null;
protected $_customerGroup;
private $logger;
protected $_customerRepositoryInterface;

/**
 * @param \Magento\Framework\ObjectManagerInterface $objectManager
 */
public function __construct(
    \Magento\Framework\View\Element\Context $context,
    \Magento\Framework\ObjectManagerInterface $objectManager,
    \Psr\Log\LoggerInterface $logger,
    \Magento\Customer\Api\CustomerRepositoryInterface $customerRepositoryInterface
){
    $this->_layout = $context->getLayout();
    $this->_request = $context->getRequest();
    $this->_objectManager = $objectManager;
    $this->logger = $logger;
    $this->_customerRepositoryInterface = $customerRepositoryInterface;
}

/**
 * @param \Magento\Framework\Event\Observer $observer
 * @return void
 */
public function execute(EventObserver $observer)
{
    $this->logger->info(' --jafar123--  ');
    $event = $observer->getEvent();
    $customer = $observer->getCustomerDataObject();
    $customerId = $customer->getId();
    $customer->getFirstName();
    $customerData = $this->_customerRepositoryInterface->getById($customerId);
    $customerData->setFirstName('jaf');
    $customerData->save();
    return $this;
}
}