Error "Playground Communication: NetworkError" when trying to run examples in browser #228

Not the most important issue, as one can just run examples with their local JRE, but the book indicates you are supposed to be able to run the examples embedded in the pages of the book, by clicking a "play" icon at the top of the code boxes.

However, when I do this in Firefox, I get an error:

"Playground Communication: NetworkError when attempting to fetch resource."