Is the code you posted the exact code you’re using? If it is then you have a typo in your field name
<?php
$Firstname = $_POST['acf']['field_5968834df2ac1'];
echo 'data' . $Firsname; // this is different than the line above.
?>
When are you attempting to get these values and save them? Is it in an acf/save_post action? https://www.advancedcustomfields.com/resources/acf-save_post/
