I've been using Mint for a few months without a firewall and haven't had any problems as yet. Is it actually necessary to use a firewall at all or have I just been lucky? I don't trust the GRC Shields Up scan data because the results for my Windows Vista laptop and my Linux Laptop seem to be identical. The former has security coming out of its ears and the latter none at all.
I've also read that Linux has a firewall built in but I don't know how true that is.