If you are receiving a CORS (cross origin request sharing) error such as:
XMLHttpRequest cannot load file:///mapbox.js/assets/data/states.geojson. Cross origin requests are only supported for protocol schemes: http, data, chrome, chrome-extension, https, chrome-extension-resource.
This means that the file you have opened is attempting to load external data, either from a relative or absolute URL:
var statesLayer = L.mapbox.featureLayer()
// Grab some GeoJSON data from a relative URL