Obviously the firewall shouldn't be making the modem crash, so it might be worth checking whether any firmware updates are available. A firmware update will upgrade the software in the modem.
If you got the modem from an ISP you should check with them.
You aren't compromising your security by disabling the hardware firewall though when you have a software firewall in place. But there's nothing wrong in having both a hardware and software firewall between you and the big bad web.