I have this problem. Some bits of history are gone except from old backups of profile directories and profiles where I've already set places.history.expiration.max_pages to some absurdly high number.

I need to do a handful of experiments to see exactly how this interacts with Sync, even though I've (foolishly) already synced the important profiles. I'd hope that the cloud copies of the places database just keeps growing, but in any case, I'd rather combine them all offline anyway.