I absolutely love this idea. The sample apps look cool, but I can't get a new app to work. When I click the '+' button to make a new app, the editor shows me

I absolutely love this idea. The sample apps look cool, but I can't get a new app to work. When I click the '+' button to make a new app, the editor shows me a hello-world index.html. Clicking "Run" results in a blank white pane regardless of whether I target a new tab, or a side-by-side frame. I get the same behavior on a downloaded copy and online. I'm running Firefox on Windows.