Posting from my iMac from 1999 using Classilla, haha!
I managed to install Pro Tools 5.2.1 and the USB driver you provided thanks to your gracious and helpful comments. I am able to load up Pro Tools. In case this helps others, you have to install everything that looks optional, including Quicktime, or Pro Tools will spit out an unrelated "disable virtual memory" error. My only remaining issue is that it's not finding my Mbox.
I get an error "The software needed to use the device "Mbox USB Audio Device copyright 2001 Digidesign" cannot be found." when I plug it in, this is after visually confirming that the USB extension is in my Extensions folder. It shows up in the System Profiler under that same name. Firmware appears to be 0.20 in the Profiler, aka ver 20 which seems like it should be fine. The Digidesign Firmware Updater doesn't recognize that I have an Mbox plugged in. Pro Tools doesn't recognize that the Mbox is connected and gives me an error saying to connect an Mbox peripheral when I try to enable record and start recording.
I've tried both the original USB driver and the updated one you provided. I've tried using a different USB slot. I have tried restarting with the device plugged in, plugging in after restart, plugging in after booting up Pro Tools. I read the readme that came with the driver and tried following its instructions exactly. I'm not sure what's going on. It's like it isn't connecting the driver in my Extensions folder to the device I'm plugging in, which is an original Mbox 1 in all its stand-up blue-and-grey glory. Is it possible mine is too new? I believe it actually came in a bundle with Pro Tools LE 6. Is there any way to make my computer realize that I do indeed have an Mbox plugged in?
Thanks again. I'm surprised that I got this far but I'm not sure if I'll achieve my dream of an iMac-based recording rig.