Looks like a few days ago GMail Notifier for Firefox 3.5.3 stopped working in terms of auto login. The author has made a patch available and is still working on the 0.6 edition (release date not yet available).

You can download the patch from here:

http://longfocus.com/firefox/gmanager/releases/gmanager0575b2.xpi

Forum where the topic is being discussed:

http://forums.mozillazine.org/viewtopic.php?f=48&t=332634&start=2475