The sudo command, which lets one execute a command as another user is what I think you might be looking for. As I understand it sudo would allow you to start Firestarter, an iptables front-end, at the launch of the operating system automatically. I wish that I could help further but I am merely a novice and can only recommend to you the Firestarter Problems on Bootup
By the way, GNU/Linux Mint
looks like an interesting Debian-based distribution, I was not previously aware of it.