Hey,
It could work in firefox. Its just that we only test it in Chrome, because we have limited resources.
We love Codeacademy and have been thinking about building such a thing on circuits.io instead of tutorials. It's a lot more fun! We will have it one of these days.
You know what I’d love? A Codecademy for electronics. There are online courses and such but none seem to be as casual and easy to get into as sites like Codecademy and TryRuby.