It very well may be. But have you tried a blank profile?
http://trac.arantius.com/wiki/BlankProfile
Creating a new script so I can paste the code in doesn't work either.
Windows XP Pro, latest FF.
I don't think that's the same. Have you read:
http://www.greasespot.net/2005/12/troubleshooting-064.html
(And is there anything in your Error Console?)