When you open a forum folder, go to the bottom of the displayed list of topics and you'll find a button labeled "More options." Click on that, then go to "Topic filter" and choose "Hide topics I've read." Click on "Apply" or "Apply temporarily" at the bottom.
Or you can go to the top, and on the right hand bar is a box titled "shortcuts," that has links to "Posts you have not read," among other things.
Alternately, if you'll look at the top of a thread, and click on "newest to oldest," the newest threads will show up, that way you don't have to scroll. Also, on the right hand side is a column with several headings. Scroll to the one that says "Active threads" and you can see which ones have recent posts.
