Clearing of the new post Symbol

This is a issue (not necessaraly a problem) with the forum software that this forum is based on. It issues a sesson id in your cookie to remember your last visit. If you browsed away or closed browser window, it will only show posts from the end of your last session until current. It avoids the need for a "mark this forum read" button, but can get annoying.
I think that the newest version of the PHPBB software eliminated this issue, but with the heavy customization of this board, upgrading it would be a nightmare.
I'm not sure how to resolve this issue myself. I would prefer it to be a thread based system, rather than a session based one myself.