The first part regarding Firefox is not true. Calculation is completely asynchronous in the IPython notebooks, i.e. if you run a cell that doesn't output anything your browser is idling.
Even if it has one pretty large output it works quite well. However, if you `print` each intermediate result and have a few hundred of those you get a problem indeed, there is a need for some kind of overflow protection here.
Great for previewing graphs and copy pasting and executing things out of order.