The answer is either to use the back button and then reload (command+R) or exit a topic via the breadcrumbs.
I'm afraid you have become used to what could be referred to as a bug or very inefficient software and it will take you a little while to adjust.
You can clear any Mac browser's cache but this is not a solution here.
As a suggestion, open the oldest topics first from bottom to top, using your back button to return to the forum as you are used to. After you have looked at 4 or 5, just reload. Start again from the bottom to the top.
An other alternative would be to use the previous and next topic links at the bottom of each topic.