My network has the same problem
I run an art college’s data network, usually 50-100 people connected via AFP to OS/X 10.5.8 server. Over the past week we’ve had our AFP service go bananas and create a 40TB+ file in random users’ Firefox Profile directory spaces. It’s exactly as you describe.
Interestingly enough, last year our Apple Sales Engineers, while ‘helping’ us trouble-shoot the now-well-published run-away AFP issue, convinced us to upgrade our AFP/OD server. We did, our problems became much lighter in frequency but now this Firefox 3 thing has surfaced.
Have you heard any word from the Mozilla team? Has Apple contacted them or vice-versa? We’ve elected to dump Firefox for now but have had to replace it with a compliant browser other than Safari because of two large-scale collaborative server system our college uses.
Thanks for reporting the issue.

