UBB does not perform post tracking. It is time based, using the cookies. I've posted a couple of times without incident on your test board, and the session indicators seem to be behaving. I have not been logged out either. I'm using the latest Firefox nightly builds (I believe it is Firefox 1.5 Alpha 2).
You might want to start surveying which browser/os combinations are having the issue. It could also be related to privacy software such as Norton Internet Security fudging with the cookies.
Another thing for them to try is to goto ubb=dumpcookies and see if they have proper cookies. You should have a login, ubber, and session cookie.
