USB 3.0 would replace Firewire.
I’m not convinced that Firewire is on its way out yet. Plenty of high end equipment still relies on it. The high end camera I used to make my final project at university (a Sony model which is regularly used in television and movies) used a Firewire hard drive because of the read/write speeds.
Until USB3.0 becomes mainstream, you’re still better off with Firewire800, although you could also go with external SATA. Ultimately if you are only going to be using it for backups, and if you aren’t doing a full backup each time, Firewire is probably your best bet (I theorize that due to Firewire’s longer existence time, the prices for components would be lower than SATA which is newer, but have no actual numbers to support this theory).