I suspect I know the answer, but no harm in asking, I guess.
I was a heavy user of "next topic" to step through unread posts.
As far as I've discovered the only somewhat similar action is to use the browser back button to return to the list of topics, then click the next topic.
You can also type u as a keyboard method of going back and g followed by u to go to unread or g followed by l to go to latest. Personally I find the keyboard shortcuts very handy.
(In order to even consider using them I have to turn off Search for text when I start typing in Firefox. I don't know yet whether I want to do that, but need to better understand the keys before I can make that decision).
From within a thread, the key combination u k o does provide a result similar to the old Next topic link, albeit by explicitly going back through the list of threads. I'm still evaluating it compared to browser back button or backspace key followed by clicking next wanted post.