Could you provide some more details. What version of Flash Catalyst do you run, on what OS, and what version of Firefox? Do you run them with default or custom browser security settingns? Does your browser run other web pages containing Flash? Can you publish the project and open the local copy of the published html in a browser? Do you get this error at all times when you run FC projects?
Hum. I am assuming you can run Firefox without issues on various websites? If you view other Flash content on the web are you encountering errors? If so, maybe the Flash plugin is wither out of date or has been corrupted. Does it happen if you have FF already open, the manually open the generated Main.html file?
Can you change your default browser and give that a try?
I am not a Windows guy, so these are just my best guesses.