Topics stay new until you read them where before they showed only those posts that are new since the last time you logged in. I think.
The 'Mark all posts read' button is found above the right-most column in the folder view.
You can right click the mark all posts read link and create a bookmark in firefox from the link. Put the bookmark on the bookmark toolbar next to the maxpc bookmark (or the porns r 4 us button if you prefer). You can mark all posts read with a simple click on the bookmark shortcut. Doesn't matter what forum you are in, you don't have to return to the index. Shouldn't have to do that since every other site I've been to lets you mark all posts read from any page. At least it's now possible to mark all posts read. Used to be you could click to mark them read, then check a few minutes later and find threads whose latest posts were from weeks earlier.