How to use a node.js back-end server to interact with the front-end and a mongolab database?

I am building a website with a simple jquery/html/css front-end and a node.js server back-end. If my front-end has a function to request a user's information from the server like so:

function requestUser(email, password) {
    xmlhttp = new XMLHttpRequest();
    xmlhttp.open("GET", "http://localhost:8888/getUser/" + email + "/" + password, true);
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
           console.log(xmlhttp.responseText);
        }
    }
    xmlhttp.send();
}

and my node server looks like this:

var http = require("http"),
    mongojs = require("mongojs"),
    fs = require("fs"),
    url = require("url");
    express = require("express")
var server = http.createServer(requestHandler);
server.listen(8888);
var uri = "mongodb://<dbuser>:<dbpassword>@ds036698.mongolab.com:36698/alirodatabase";
var db = mongojs(uri, ["Papers", "Users"]);
console.log("node server running back end of app");
function requestHandler(request, response) {
    //request for user is .../getUser/<username>/<password>
    var path = url.parse(request.url).pathname;
    var details = path.split('/');
    if(details.indexOf("getUser") != -1) {
        console.log("recieved request for user");
        var user = db.Users.find({"email": details[details.indexOf("getUser") + 1],
                                  "password": details[details.indexOf("getUser") + 2]});
        user = user.toArray[0];
        response.writeHead(200, {"Content-Type": "text/json"});
        response.write(JSON.stringify(user));
    }
    else {
        fs.readFile("./index.html", function(err, file) {
            if(err) {
                return 
            }
            response.writeHead(200, {"Content-Type": "text/html"});
            response.end(file, "utf-8");
        });
    }
}

why isn't it working? I get a 'mixed content' error from firefox when I try to request from the server. I've heard that github pages only serves static sites, but heroku doesn't support html/css? Help and thanks.