Okay, I use this feature a lot on the SA forums (which runs a really hacked-up version of VB) to keep track of long threads. Basically the 'go to first new post' button in that takes you to the first new post (in the case of an unread thread, the first post in that thread), then bookmarks you, so if you've read only the first page, clicking that button again takes you to the second page.
Here it works the first time, taking you to the first page in a post, but then it marks the whole thread as read and the button stays grayed-out until a new post is made after your last visit.
Is this intentional? Because it's pretty useless as a bookmarker since it's most useful in those 20/30/100 page threads where you've read, say, 10 pages in and want to keep your progress without having to leave a tab in your browser open with that page perpetually.