Somewhere during the 3.0 betas, firefox started generating nasty amounts of disk activity. The activity is occasionally so high that the machine becomes unusable for a while. It is something metadata related as it's kjournald that gets stuck in D state. The filesystem is mounted with noatime, so that should not be the problem. I'm not sure how to properly trace it beyond that.
*** This bug has been marked as a duplicate of 439908 ***