Hi,
the “firewall” in Qubes OS actually consists of a VM named sys-firewall, running Fedora by default. It is heavily customized and integrated into the Qubes OS ecosystem (qrexec et al.), which is why IPFire is neither a suitable replacement for it nor would provide additional security in this context.
Please refer to the Qubes OS documentation for further information, and/or ask at the Qubes OS project forum or mailinglist in case of more detailled questions regarding it.
You are, however, free to protect the device running Qubes OS by putting an IPFire machine in front of it. 
Thanks, and best regards,
Peter Müller