I have deleted a rule but not applied the change.
would like to undo the deletion.
How can I do that?
You can download an old backup, and unzip it locally (the .ipf file is really a zip), and then look at the file /var/ipfire/firewall/config for your old rule (or replace complete file in the firewall). There is no “undo” function; the UI edits the forementioned file, which is then used as a template to apply iptables rules.