Zero Day Initiative have advised Mozilla of a new security vulnerability that can allow execution of arbitrary code with the credentials of the user. This affects both Firefox 3 and Firefox 2. No further details are currently available. It is advised to use an alternate browser to mitigate exposure until a fix has been released.

Original announcement here: http://dvlabs.tippingpoint.com/blog/2008/06/18/vulnerability-in-mozilla-firefox-30