The file that stores the wui firewall rules settings and provides them to iptables is
/var/ipfire/firewall/config
You would need to edit this file to remove the errant rule but you would likely need to change the order number at the beginning of the line for all rules that came after it is the last in the series.
Before editing the file make a backup copy so you can revert if something goes wrong.
There are also files outgoing and input in that directory for the Outgoing Firewall Access and Incoming Firewall Access tables on the wui page.