Since the macbook pro I have only has 1 firewire port, how dependable is a firewire hub or using the thunderbolt port for external hard drives?
Firewire doesn’t need a hub, devices can be daisychained.
Thunderbolt is in it’s infancy and expensive, firewire is proven and more than capable.
I wanted to get an external hard drive but I only have 1 firewire port and already use my recording interface in that port. The external had drive has 2 firewire connections on it…can I plug the interface into the external hard drive and the hard drive into the mac?
Yep, that’s the idea. You can buy a firewire hub, but that does exactly the same thing, they’re just daisychained ports. Do check if either the harddrive or the audio interface have 2 firewire ports. If not you will obviously need a hub.
Actually both the interface and the hard drive have 2 firewire ports
Well there you go, most firewire devices do 
Does it matter which way? The interface to the hard drive or the hard drive to the interface?
Does it matter which way? The interface to the hard drive to the mac or the hard drive to the interface to the mac?
Well, in theory there is an ever so small delay from the extra bit of cable so I’d say put the audio interface closest to the PC, although I doubt you’d notice a difference.
One thing to note is that if you’re using devices that draw their power from the firewire port (no external powersupply) you may find that the firewireport on your mac cannot supply enough power to run all devices.
Thanks for the help!