I too think there is something going on with Spybot 1.6 Immunization. I spent close to half an hour trying to make it work with FF 2.0.0.14 & 3.0.1 on a brand new formated disk that I installed Win XP SP3. Spybot was the second program I installed after Norton.

I un-installed FF and Spybot 2 times each with lots of reboot in between but I could never get it to immunize the FF part. I could not even find the hostperm.1 file...

Then by magic (I swear) I saw the FF part had been immunized. I do not know what I did differently.

I know I'm not giving you much to troubleshoot but maybe you can test out stuff anyways.