well google will notify you of popups being blocked on a page and then you can choose to allow popups on the page. firefox i am unaware of how their popup feature works.
for those using XP, Service Pack 2(free and recently released), and Window's New Pop Up Blocker for IE then it will display a notification telling you Windows has blocked X popups and how to retrieve information about them
anyways a popup login wouldn't be too bad and maybe make a user-end option if their member if they want to see it