Whonix runs TOR in a separate VM from your browser/user space. The idea is that even if you get hacked they don't get your IP address since they can only access the internet through the gateway VM that pushes all traffic through TOR.

Ideally you'd make sure the one responsible for going through tor isn't Firefox, too; IE at the very least a wrapper such as tsocks when running it or even better, a VM containing the browser and the entire VM connecting only through tor.