(wish-I-could-trust-the-)Edit: nevermind. After harder searching, I see now that there's already an option to sort the threads by Post Date, revealed when you click "options" below the list of posts. And it actually works, too.
I have another request, though: please disable the periodic refresh of the topic list, or make it optional! Because when I'm browsing through older WTFs, I like to open topics in new windows, and so my topic list window will periodically be taken back to the first page by this refresh.