CNET reports that Secunia has uncovered a "highly critical" security flaw in Firefox 2.0 and later versions, which can allow a malicious attacker to gain remote control of your computer.
When Firefox first came out, a compelling reason to switch from Microsoft explorer was that Firefox was more secure. At the time, it's userbase was extremely small and dominated by very geeky people. As Firefox gains mainstream adoption, it seems to me that bad new like this had to happen. If users are moving from one browser to the other, then the bad guys are pretty certain to follow.