FYI I think I'm having the same issue as Retropin.. both the "Show unread posts since last visit" and clicking the unread tab show the same limited number of posts. Prior to the update I had a lot of unread posts. I had never clicked "mark all unread posts as read", as once i'd read everything I was interested in that was actually "new", I would go on to read the older unread stuff.
It's as if the update marked everything as read, so now I the only unread posts I see are posts since my last visit, rather than everything that is unread.
I'm also using Firefox (v3.6.12).
