I think you just indicated the answer when you talked about cache objects being partially written to disk.
Do you know if firefox opens any of its cache-related files as Non-buffered? Looks to me as though it might do, just to reduce problems. That's fair - you need the most robust code possible and there's lots of people who do dumb things.
Yup I know about about:config. I have a couple of changes in there, mainly so that Outlook can handle the mailto's. Its not paging that's the issue - I can monitor that fine. The program is disk bound for me on the 5 machines I use.