I had the same problem with iMovie 1, but was able to copy a disk image from an iMac system restore CD. (iMovie 2 installed okay). Same basic config, G3 w/ Orange Micro combo board.
I haven't tried installing DiscBurner yet. Apple sometimes list "built-in Firewire" as a system requirement, and did for both versions of iMovie. On some of their installers, it does a lookup to see if your machine is on their approved list and if not, too bad.
It's not like I'm going to go out and buy a new Mac if their latest software doesn't like my machine! I just find a getaround or another app. Dumb.