This might seem random but there is a logic behind it. When viewing a forum Simple:Press saves a bookmark to help the user get back to the page where they came from. Lets say you are paging through all your topics and are on page 6 and you open a topic and realize that that was not the topic you were looking for. If you click the breadcrumb Simple:Press will use the bookmark created to try to make a seamless jump to the paging view.
But in your case I believe that jumping between browsertabs and topics makes this more confusing that helpful.
Personally I think that linking to the first page is more logical since I would use the back button in the browser to get back.
Thanks for pointing this out!
Joakim @ Simple:Press