Launchd item to add a firewall rule via ipfw.

com.talkingmoose.addipfwrule.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
	<string>com.318.ipfwadditions</string>
	<key>ProgramArguments</key>
	<array>
		<string>ipfw</string>
		<string>add</string>
		<string>9997</string>
		<string>allow</string>
		<string>ip</string>
		<string>from</string>
		<string>any</string>
		<string>to</string>
		<string>any</string>
		<string>via</string>
		<string>en0</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
</dict>
</plist>

Save this file to /Library/LaunchDaemons to run at every startup.

You can add multiple rules by inserting a semi-colon between sets of strings or by creating a new file for each rule.

Be sure to adjust permissions on the .plist file: owner:root, group:wheel, 644.