about:config and change the value for
But it will not stop your IP from being revealed in case your VPN connection drops. It might decrease the risk somewhat, but not eliminate it. After all, the VPN might go down mid page load (and pages can keep loading things for quite some time in the background).
What you need is a "kill switch", i.e. something that shuts down all connections as soon as your VPN goes down. Quite often, VPN software comes with one included.