A friend recently bought a new G5 Mac and got broadband internet service for the first time. At the recommendation of the retailers, she leaves the Mac on and online 24/7. But she has no firewall (hardware nor software). Now, as a PC user, having an always-on internet connection without a firewall is, alas, just waiting for trouble. But what about with a Mac? Is a firewall necessary with an Mac? If so, what about just a software firewall? Any recommendations for cheap or free programs? Thanks!