Profile Sync Daemon is a tool to move the web browser profile to RAM, using tmpfs, which has the advantage of reducing disk writes (useful for SSDs, some would say), increased speed and responsiveness.
Since RAM content is cleared on reboot, Profile Sync Daemon automatically syncs the profile back to the physical disk using a symbolic link and rsync at a given time interval. Further more, the profile is synced back to the hard disk before you shut down or restart your computer, so no profile data is lost.