I'm guessing that in order to use the tracking site you need to be logged in. When you are redirected to the main site, can you tell whether it's because you are not logged in?

Firefox usually retains websites' login cookies, but if you have changed the permission sites have to set cookies, or if you set Firefox to clear history, or if you have add-ons that manage privacy or cookies, or if you have external software that cleans up browser cookies, then the cookie might not be retained. It's a lot to check, I know...