I don't dispute that it is better than IE. But that isn't saying much.
According to [link|http://secunia.com/search/?search=firefox|http://secunia.com/s...h/?search=firefox], all that has been reported recently are denial of service attacks. But just over 3 months ago a whole ton of "execute arbitrary code" bugs were found in firefox, and I don't doubt that there are plenty more.
Cheers,
Ben