Is there an equivalent to CTRL+C in IPython Notebook in Firefox to break cells that are running?

You can press I twice to interrupt the kernel.

This only works if you're in Command mode. If not already enabled, press Esc to enable it.

I could be wrong, but I'm pretty sure that the "interrupt kernel" button just sends a SIGINT signal to the code that you're currently running (this idea is supported by Fernando's comment here), which is the same thing that hitting CTRL+C would do. Some processes within python handle SIGINTs more abruptly than others.

If you desperately need to stop something that is running in iPython Notebook and you started iPython Notebook from a terminal, you can hit CTRL+C twice in that terminal to interrupt the entire iPython Notebook server. This will stop iPython Notebook alltogether, which means it won't be possible to restart or save your work, so this is obviously not a great solution (you need to hit CTRL+C twice because it's a safety feature so that people don't do it by accident). In case of emergency, however, it generally kills the process more quickly than the "interrupt kernel" button.

Here are shortcuts for the IPython Notebook.

Ctrl-m i interrupts the kernel. (that is, the sole letter i after Ctrl-m)

According to this answer, I twice works as well.