Same here, Firefox 36 / Ubuntu 14.04. The error is because the page is HTTPS and it is requesting assets from an HTTP page (http://vector.mapzen.com/...) "Blocked loading mixed active content".

I'd love to see the demo the rest of you are seeing but not enough to disable this security setting. I'm also using the NoScript add-on but temporarily allowed mapzen.com which had no effect).

I'm always happy to see a site using HTTPS but people need to understand that means no more HTTP assets - this is really basic and cannot be overlooked. If you want HTTP great, stick with it and quit dabbling with HTTPS.