In Firefox, when I click "Go to the new posts" it, as usual, brings up threads that I haven't read yet. However, it seems to only mark them as 'read' when I click "go to the new posts" again and not when I click the 'go back one page' button. It even marked a thread I created last night as unread. This never happened in Edge. I'm using Win 10 Pro

Firefox stores a cache of the previous page when you press the back button rather than reloading the page. This is configurable.

That's exactly what this is. Most browsers will load the previous page from a cache when you press back; it's why you can press back and still have text in a text box.

Since this "bug" is solved, locking the topic.