Firefox is not retaining cookies after the session is closed.

[SlyMaelstrom]

Hmm... try deleting cookies.sqlite in your profile folder:
<HD>:\Documents and Settings\<name>\Application Data\Mozilla\Firefox\Profiles\<your profile folder>

Heh, I took it one step further and just uninstalled and reinstalled Firefox and it fixed the problem.

I attempted to duplicate the problem by installing the updates on a desktop running Vista x64, but after it installed the updates, the problem didn't occur. This makes me feel something else caused it. What it was, I have no idea... but it's strange that it happened exactly on a reboot where Vista installed "Important Security Updates." I'm really not certain what the problem was, now.

Anyway, all is good... I just have to import my bookmarks and reinstall a few essential add-ons and I should be good to go. Google Chrome is nice and all, but I'm happy to go back to my Firefox.

It appears your solution of deleting cookies.sqlite would be the correct one after doing a little more researching.