That is according to Secunia, which tracks security vulnerabilities.
Sample exploit code is already available online, so while there aren't yet any reports of active attacks against this new flaw, there soon could be. Such an assault would likely take the form of a poisoned web page that uses behind-the-scenes attack code to trigger the flaw.
See Brian Krebs' post for instructions.
Firefox 3.0 users who haven't yet upgraded should not be vulnerable to this flaw, it is said, and will not find the setting that Krebs describes.