Why when I build Html5 can Firefox not find the file "MyProject.html"?

Yeah dont worry I know the feeling.

Okay looks like your having the exact same problem I was having. Even though .py files are set to open automatically with python it does not seem to be passing the args over correctly.

I fixed this by opening HTML5Platform.Automation.cs (Under Programs/Automation in VS). and going to Line 36 and changing the string to:

/c python {0} {1} --preload . --pre-run --js-output={1}.js

Then doing a rebuild in Frontend, hopefully the SwingNinja.data and SwingNinja.data.js appear correctly.