talkinggoat,
Did you previously have Firefox 2 installed on the PC, and if so did you completely uninstall it before you installed Firefox 3, and by that I mean manually remove any left over folders and files after the un-installation process.
If you look at the immunisation screen, in the Firefox group, are there two of each catagory. If there are, could you see whether a file named hostperm.1 is in your browser profile folder - if so try deleting it - it belonged to Firefox 2 - Firefox 3 uses a file called permissions.sqlite.
If this fixes the problem, post back and let us know, if not could you please provide some details about your operating system, CPU type, etc, and perhaps also some time frame details as to when Spybot 1.6 was installed relative to the installation of Firefox 3, and whether there was an earlier version of Spybot on the computer.