If your cache is set small, the OS will constantly have to write over existing cached info as different apps load up and operate. As far as Firefox being the worst culprit, you can adjust your FF cache and how pages are updated in the FF preferences. On my system, FF is set to have a 1Gig cache. I rarely see FF writing to cache unless I've recently cleared it.
I'm just tossing around suggestions here, Hugh, because I really haven't a clue as to what is accessing your disk so often. Have you run top yet to see what activity is going on?
Sometimes, top can be very helpful in figuring out what is eating up clock cycles and resources on your system.