<div id="firstName"></div> <!--Fill this one if the other one is empty -->
<span merge-tag="{{lead.first_name}}"></span> <!--Check to see if this one is empty before filling div -->

// URL Parser
function getParameterByName(name, url) {
    if (!url) url = window.location.href;
    name = name.replace(/[[]]/g, "\$&");
    var regex = new RegExp("[?&]" + name + "(=([^&#]*)|&|#|$)"),
        results = regex.exec(url);
    if (!results) return 'blank';
    if (!results[2]) return '';
    return decodeURIComponent(results[2].replace(/+/g, " "));
}

//First Name from URL
var firstName = getParameterByName('first_name');

//Function to add first name to div if span with merge-tag="{{lead.first_name}}" is empty.
function addName() {
    if ($("[merge-tag={{lead.first_name}}]").is(':empty')) {
        document.getElementById("firstName").innerHTML = firstName.toString();
    }
}

window.settimeout(addName, 5000);