bk-wc-checkout-thankyou-title-personalize.php

<?php
function rp4wp_title_order_received( $title, $id ) {
	if ( is_order_received_page() && get_the_ID() === $id ) {
		global $wp;

		$order_id  = apply_filters( 'woocommerce_thankyou_order_id', absint( $wp->query_vars['order-received'] ) );
		$order_key = apply_filters( 'woocommerce_thankyou_order_key', empty( $_GET['key'] ) ? '' : wc_clean( $_GET['key'] ) );

		if ( $order_id > 0 ) {
			$order = wc_get_order( $order_id );
			if ( $order->order_key != $order_key ) {
				unset( $order );
			}
		}

		if ( isset ( $order ) ) {
			$title = sprintf( "Thank you, %s!", esc_html( $order->billing_first_name ) );
		}


	}

	return $title;
}

add_filter( 'the_title', 'rp4wp_title_order_received', 10, 2 );