I use firefox and use tabbed browsing a lot. I like to be able to have various threads open and flick between them, mostly reading, sometimes posting. When you have the threads set out so that all posts are on one page, you can have to scroll up for several seconds to get to the top of the page.
With the viewport bar (when it was working correctly) I could be at any point in a thread, then open a new tab to check 'I'm on' or 'Active', etc. It also meant that I didn't have to 'lose my place' in the thread (I know there are bookmarks, but I'd use those if I actually intended to leave the thread, a la 'closing a book', rather than flicking between threads, a la looking up from a book to the TV and back again...)
I miss it, but would be fine if you just added those options that came with the viewpoint bar above the 'start new thread' bar at the bottom of each thread (before the 'add your message here' text box).