popupContent demo

HTML

<link rel="stylesheet" href="https://api.tiles.mapbox.com/mapbox.js/v1.6.3/mapbox.css">
<script src="https://api.tiles.mapbox.com/mapbox.js/v1.6.3/mapbox.js"></script>
<div id="map" style="width: 550px; height: 600px"></div>

JavaScript

/***  little hack starts here ***/
L.Map = L.Map.extend({
    openPopup: function (popup) {
        //        this.closePopup();  // just comment this
        this._popup = popup;

        return this.addLayer(popup).fire('popupopen', {
            popup: this._popup
        });
    }
}); /***  end of hack ***/


var layerOne = {
    "type": "FeatureCollection",
    "features": [{
        "type": "Feature",
            "properties": {
            "name": "Utrecht",
                "popupContent": "Pudebit te, inquam, illius tabulae",
                "marker-size": "large",
                "marker-color": "#de0c14"
        },
            "geometry": {
            "type": "Point",
                "coordinates": [
            5.11962890625,
            52.11325243469631]
        }
    }, {
        "type": "Feature",
            "properties": {
            "name": "Rotterdam",
                "popupContent": "Quarum ambarum rerum cum medicinam pollicetur",
                "marker-size": "large",
                "marker-color": "#de0c14"
        },
            "geometry": {
            "type": "Point",
                "coordinates": [
            4.4879150390625,
            51.91716758909015]
        }
    }]
};

var layerTwo = {
    "type": "FeatureCollection",
    "features": [{
        "type": "Feature",
            "properties": {
            "name": "Eindhoven",
                "popupContent": "Quae fere omnia appellantur ",
                "marker-size": "small",
                "marker-symbol": "circle",
                "marker-color": "#de0c14"
        },
            "geometry": {
            "type": "Point",
                "coordinates": [
            5.47119140625,
            51.45400691005982]
        }
    }]
};

var layerThree = {
    "type": "FeatureCollection",
    "features": [{
        "type": "Feature",
            "properties": {
            "name": "Arnhem",
                "popupContent": "luxuriae licentiam pollicetur"
        },
           ...