What's uploaded to the arcade is just the event sheet structure. The arcade concatenates this with the whole runtime.
I could maybe solve the problem by doing a new export, but I'm not even sure where is the last capx or if it would still work (nope I'm not lazy... not at all!)
So... meh... maybe on one rainny day.
Looking at it closely, the problem comes from the fact that firefox changed they renamed "noteOn" and "noteOff" function of the AudioContext's BufferSource object for "start" and "stop".
So I guess I'm probably not the only one who should reexport their game. Or maybe better, scirra arcade should update its code to correct all the bugs in one place =) (dunno how hard it would be)