I would note, for the curious, that you can also quote a heredoc (for large blocks):
sudo bash -c "cat <<EOIPFW >> /etc/ipfw.conf
<?xml version=\"1.0\" encoding=\"UTF-8\"?>
<plist version=\"1.0\">
<dict>
<key>Label</key>
<string>com.company.ipfw</string>
<key>Program</key>
<string>/sbin/ipfw</string>
<key>ProgramArguments</key>
<array>
<string>/sbin/ipfw</string>
<string>-q</string>
<string>/etc/ipfw.conf</string>
</array>
<key>RunAtLoad</key>
<true></true>
</dict>
</plist>
EOIPFW"