First of all, why don't you buy a class c. That answer is just as pertinent as those others that don't answer your question and seem to have some type of agenda.
I only found a few references to your ACTUAL question and they seems to say that the issue with with your virus program. Try disabling your virus program and start up FF. If you don't get the msg, you will need to look into changing an option there.
Also, are you running as Admin? if not, right click the icon and select run as Admin.
there are references to a corrupt file, but since you are past the install stage, I don't think that is the problem.
If all else fails, uninstall FF and reinstall it, and keep a close watch on any boxes that need to be unchecked or checked.
And remember, get a class c to really fix the problem.