I wanted to thank you again for investigating so quickly and indicate that fixing the app code to avoid the JavaScript error resolved the situation for both Firefox and Chrome.
This doesn't says exactly why Safari worked through the error, while Firefox and Chrome didn't (neither does it say what the "right" behaviour should be for any web browser in that case), but at least I hope it will it gives a good precedent for the issue and solution if anyone else runs into a similar issue in the future.
The one-liner fix can be read here: https://github.com/csoneson/iSEE/pull/274/files
I'll be submitting the app for the contest shortly now! 
Best wishes,
Kevin