For me, on this site, Firefox doesn't do anything at all. If the page you're on has some type of Ajax script running, then whatever it downloads will cause disk access.
jdb2 is the ext4 journal. It would be normal for this to show up whenever some other program writes to disk. However, there does seem to be an ext4 bug with behavior that matches your description: https://bbs.archlinu...pic.php?id=113516. No clean cause there, but a few workarounds for specific users.
flush popping up is normal whenever something has been sent to disk. This is most likely triggered by the Firefox writes.