Unresponsive script is different then slow script, flash offers the same unresponsive script warning when a thread gets caught in an infinite loop, and neither environment knows how to deal with a slow script.
Well… Browsers use the same thread for UI updates and JS execution, so every time[1] Javascript is executing your UI is blocked—which means that if your script runs for longer than ~0.2s UI becomes noticeably unresponsive. So IE will decide that your script is unresponsive after 5mil statements, Firefox—after 10 seconds, and Safari after 5. Opera seems not to care
[1] Unless your are using webworkers. Those are executed in separate threads.
