Nondeterminism is every place where information can enter your program from the outside world. It's random numbers, the system clock, data off the internet, an

Nondeterminism is every place where information can enter your program from the outside world. It's random numbers, the system clock, data off the internet, and input events from the user. What it sounds like is that Mozilla has come up with a debugging environment that bottles all that nondeterminism up into a neat little package so that it can replay a specific run of your program any time you want.